Looking for an ebook reader (hardware) which doesn't hold a proprietary OS

I've looked into this in the past and settled on Kobo. You can disable the telemetry and never use the the Rakuten account part and have a very good ereader... And you can install the open source KOReader software.
https://github.com/koreader/koreader
MobileRead forums and wiki are a good resource for ebook stuff.
For example, a breakdown of the hidden configs on Kobo devices https://wiki.mobileread.com/wiki/Kobo_Configuration_Options
- 1 on this. Kobos actually use Linux under the hood. And although the default UI is proprietary, it's super easy to install KOReader.
  You don't even need to hack into it some custom firmware, just a sideloader, which normally doesn't break even if you actually updated the base firmware.
  Here the official tutorial on how to do it: https://github.com/koreader/koreader/wiki/Installation-on-Kobo-devices
  
  AFAIK every single ebook reader on the market actually runs Linux under the hood!
- My kobo is great. You never have to connect it to the internet if you don't want to. I transfer epub files to it via USB.
  
  Same here. I only buy drm-free from ebooksdotcom, and transfer them with ~~caliber~~ calibre. My kobo wifi isn't even configured.
- Kobo readers are really neat, I've been using them for over a decade and I don't remember ever using a Rakuten account or even going online with them for anything but software updates or connecting to my local library system (which Kindle can't do). I use Calibre on the desktop to manage, convert and load my reader and that's it.

Your best bet for a no BS ebook reader might be this (if it where in stock): https://pine64.com/product/pinenote-developer-edition/
- The PineNote won't be in stock until the community of developers figure out a proper Linux based OS for it. I dig through the forums occasionally and they are making progress, but it won't be soon.
  
  Yeah, the development for the Pine products take their time, but I wouldn't want to trade my PineTime for something else. Great gadget for an incredibly low price!

i can connect to my pc, pass ebooks in different formats into it and read.
Sounds like you don't need internet usage. In that case, you can use most ebook readers, including stuff with companies shit on it, and just never connect to the internet. If privacy is the concern, they can't track you if you never connect to the Internet. You just need to make sure that it has a way to transfer files through usb and can handle epubs.

I love my Remarkable 2. The company has a freemium model for its online services, but the device is lovely on its own and it’s Linux under the hood, with an active modding community delivering cool tweaks.
- Out of curiosity, what kind of tweaks?
  
  Here are a few! There was also a whole wiki, RemarkableWiki.com, for a while where users shared technical tips and tricks. It's not up at the moment and I'm not sure if it's down permanently or only temporarily. My experience has been similar to @blusterydayve26@midwest.social — I bought the device because I liked how user-modifiable the software was, but once I had it in hand I found that official development was moving briskly enough with new features and UI improvements that I've never really had a reason to mod it. I have SSH'd into the device to set it up with a few of the trickier WiFi networks in my life, though, and can confirm that it's a breeze.
- Second the remarkable. It works well enough that I haven’t even bothered to install any of the community extensions, though that would be easy since it’s just Linux and I can SSH into it to install packages.
  
  My experience has been very similar. As I say below, "I bought the device because I liked how user-modifiable the software was, but once I had it in hand I found that official development was moving briskly enough with new features and UI improvements that I’ve never really had a reason to mod it."
- It looks awesome, but doesn't seem to be that great as an ereader, though.
  
  It really awesome when it comes to reading and annotating PDFs. That’s the main reason I got it — so many e-readers I’ve tried over the years have been horrible for PDF documents and as a professor that’s like 80% of my day. For ePub documents, it’s very capable now — even if that wasn’t the case a few software versions ago. That said, the experience is a bit idiosyncratic among e-reader devices. The Remarkable basically converts the ePub to a static document so that the UI can more or less treat it as a PDF, which is a different user experience than some other e-readers. It’s not unpleasant, but it’s different.

I’ve had a good experience with an old Kindle 3G and the Calibre program. You can get the old Kindles pretty cheap and they don’t have the built-in ads and some of the slightly useless features of the new ones.
That, or like the16bitgamer mentioned, an iPad has good options for free ebook apps without ads or tracking. I found this one called eBoox that has great usability and no ads. It’s weirdly marketed as a “cute” ebooks app, but it’s honestly better than the stock one and doesn’t actually have cats or those pictures that are in the description in the actual app.

Take a look at pocketbook devices. https://en.m.wikipedia.org/wiki/PocketBook_International
- Yes, I have a Pocketbook and it runs on some Linux distribution variant. Very happy with it.

I think it depends on how invested you are in ebooks, and how much time you wanna spend on it. I would advise a Kobo if you aren't up for Tinkering or an iPad if you are flexible with the screen.
But if you are up for a challenge a Chinese ePaper Android Tablet like Onyx Boox or Bouyee, so long as you can get Google Play to work. Or a Pocket Book if you can sort out DRM removal for ebooks.
Here are the pros and cons bellow
Kobo is the easy option.
Adobe Digital Editions for non-kobo DRM, and library access. Its able to read DRM free books like you find on Project Gutenberg or Humble Bundle.
Major downside is that you can't read Amazon without effort (or a kindle serial number), book sorting kind of sucks without Calibre, and the storage size is small if you are into Comics.
iPad is the safe option
Apple Books app is convenient and can read anything. It can sync with your iCloud if you wanna so you can continue on your iPhone. And DRM isn't an issue since you can just download the apps.
but its a LCD Tablet, and no ePaper display. iTunes isn't the easiest to figure out to move books and iCloud can get verrry expensive if you are syncing comics.
Android Tablets are kind of in the same boat but...
with KOReader even an old (but not too old) tablet is viable. Side loading official apps.
OS updates are kind of hit or miss, support for older android is worse than iPad, and the devs don't put as much effort in their Android ports.
Android ePaper tablet (Onyx Boox)
Usually steals KOReader as its base, if its new probably has pen support so you can use it as a writing tablet, if it has Google Play you can get official apps
But its expensive, there is often no updates to the OS, usually no MicroSD card, and has a lot of preinstalled bloat which is hard to trust.
Kindle Tablet/fire tablet
Cons, its made by Amazon and will track your every movement.
Pros keep it offline and it can read converted DRM free ebooks converted to AZW3 via Calibre. Fire Tablets can be made into cheap eReaders with side loading. But more importantly if you do give your kindle an Amazon account you can decrypt ebooks with its serial number. So you can get cheap books on a better eReaders.
- If you go for the Kobo Clara HD (Not the newer one), you can go around the storage size by replacing the internal microSD card (It is a bit of work, yes, but it works, and is doable)
  Also, on Kobo, it is very easy to put KOReader on as well, so very much, comfy.
- Bought an android tablet, installed Moon Reader+. Never looked back
- Might be worth noting but you can jailbreak a wide array of Kindles to use KOReader opening up the device for even more file types.
- I just bought the onyx boox page and I'm not seeing much, if any bloat. It's a premium ebook reader ($250), but I bought it to replace my aging Kindle Oasis. I use moon reader pro instead of the built in reader. Google Play worked fine straight out of the box. It has a micro SD card slot for more storage as well.
  Overall I'm very satisfied with it and it is completely comparable to Amazons premium ereaders (honestly way longer battery life than my oasis ever had).
  Time will tell on OS updates, but truthfully I don't really care much about that. At least until my apps stop working.
  
  I have a Nova 3 Color and 2 like books.
  The likebooks never got OS updates, but the Nova got updates, but they were updating the default apps.
  I don't like moonreader, as I found it to be a battery hog. KOReader is my favourite and its the default reader (or a skin of it is)
  By bloat I am referring to the Onyx store which is on my home screen and is not removable.
  With that said my Nova is my preferred eReader, especially when I kill the WiFi. 2+ week battery life FTW

Boox run android, so that might fit your title.
From your post content though it actually sounds like you'd be happy with a Kobo. You can easily transfer books and you can install third party open source reading software too.
- Second on kobo, sure the OS is proprietary, but it's good enough that I forget. And as the previous comment said, you can install third party reader software. I like koreader, it pretty much replaces the entire UI of the device

+1 Kobo or PineNote

I have a Remarkable 2 as well that I love. But I also have an old Sony PRS-500 that is still kicking along and takes ePubs without any problems.
- the remarkable software is proprietary. they just run it on linux and give you root via ssh, but installing anything voids your warranty...
  
  Maybe I'm jaded / cynical, but that seems like a pretty reasonable compromise. If you want to take responsibility and install your software from scratch then nothing is guaranteed. I'd take that deal.
  
  So it's basically just a Kindle but you don't have to use an exploit to gain root access?
- Does the Remarkable do stuff if you touch the screen with your fingers? Or can I make it not do that, and only react to the pen?
  
  It does react to finger input, but it really shines with the pen. I'm not sure it is really worth it without the pen if I'm honest.

Look at OnyxBoox
- I own an Onyx Boox Air, but sadly can't recommend it as Onyx refuse to comply with the Linux kernel license and don't publish their changes. Not only is this a dick move it means custom ROMs and hacking is not likely to happen.
  
  And is illegal.
- Seconded, i love my Boox. it just runs android (with tweaks for e-Ink) and you can install what you want from the play store, it's not locked down.
  You can even install the Kindle app if you ever do want an Amazon ebook, works really well.
  It's also nice for using apps of various newspapers.
  Plus the ones with a stylus make for a great notebook.
  I wouldn't recommend the color ones, it's nice for comics but the colors just aren't vivid and it's not there yet in terms of quality.
  
  I have the first Boox Nova (color) and while it's true that the colors aren't vivid, it actually makes for a great comic book (and manga) reader. The color pallete in comics is generally limited anyways, plus the grainy low-DPI image reminds me of how comics used to look like back in the day. So ironically the limited display actually makes it a great fit for comics.
  But of course, it's not ideal if you have want to read full-color high-res content like magazines and modern webcomics (you can, but the performance isn't really that great).
  For me though, as a manga/comic reader, instead of the display, the most limiting thing I found was actually the RAM - after a long comic reading session it would run out of RAM, bringing the OS to a crawl, and forcing me to restart my apps. But it's not a huge issue, or maybe there's a memory leak in Tachiyomi. Regardless, I feel 3GB isn't enough for any large device these days.
  Still, right now, this is the best "open" ereader that you can actually buy, that doesn't lock you into any subscription (like the reMarkable tablets) or proprietary apps.

I've been very happy with my Vivlio Touch Lux 5 which is Linux based - https://www.vivlio.com/en/
- That looks interesting. Looks like they only ship within the EU though.
- I didn't know pocketbook was branded like this in France! Love them.

Is there anything that still has side buttons and no touch screen? I'm still holding on to my old kindle 3rd gen (kindle keyboard) because I abhor touchscreens on my books.
Ideally also with no backlight, or the ability to turn the backlight off.