Is it worth upgrading my e reader?

With eReaders I'd not upgrade before either:

They genuinely break.
You need a function your current one does not have.

I'm rocking a Kindle Paperwhite 11 now, but only because my previous Voyage broke sadly. :( Loved that thing with the pressure-sensitive rim.

+1 for “use it until it breaks”

Though I don’t use it as much nowadays now that I work from home most of the time, I’ve had one kindle model or another for about a decade at this point, and I used them constantly when commuting. I prefer to keep that particular device sleek and minimal, so I don’t use a case. This tends to result in it eventually getting banged up a bit after a while, but the things are generally quite robust and I tend to just ignore any purely cosmetic damage unless it’s a screen issue. I’ve gone through 2 kindles in that fashion, and I’m on my 3rd. Also, I generally only buy them when they’re on sale, and always go for the lowest memory paperwhite model (I use it for books exclusively, since I have a phone, and I will listen to audiobooks on that if I want to).

Try the kobo brand of ereaders. You have access to an online library and can download through Wi-Fi, but can also load your own books in many many file formats.

I bought the waterproof version to read at the beach, while paddle boarding, in the bathtub.

Kobo is also great because you don't even need an account. That takes away some of the convenience features OP was asking for like syncing, but I love the fact that I don't need to hand over any personal information to Kobo in order to read my books; nobody knows what's on my bookshelf but me.
- Newest version does require a sign up the first time, but you can remove it immediately
Yeah Kobos are nice and very piracy-friendly
While I really dislike Amazon I can email ebooks including pdfs and epubs direct to my kindle, I don't even need to go get it and plug it in and I can do this from my phone if I want.

If I have something really funky I can use calibre to convert and then email, or if it's really big I can always plug it in and transfer directly.

There are a lot of reasons to not like Amazon products but the kindle is actually pretty good considering who makes it.
Another + for using it until it breaks!

I am a moderately heavy kindle user and have been since the second version they shipped. When I upgrade, I usually buy the best new model available. I am skipping the one with pen support because Amazon’s text autosuggestions are absolutely the worst I have ever seen - it’s like they’re just using a random number generator and not a predictive algorithm - so my current Kindle is the Oasis.

It is so far beyond any other one I’ve owned that they’re not really comparable. The backlight is steady and even with no patchiness. The text reads cleanly with no fuzziness around the fonts. It’s comfortable to hold, and because it just inverts very cleanly and automatically it makes it trivial to hold upside down if you change hands or roll over. My requirements for a case are that it makes the device easier to hold and prop up for hands free reading in bed. Any of the origami cases should do - I think they’re all very similar in design but I’d just go off the reviews for build quality.

That said, there’s a number of kindle books that cannot be read on kindle devices because the publisher decided to prioritize the formatting over the text, and those I have to read on one of my iPads. I still prefer the kindle for text only books because it’s lighter and easier to hold.

The oasis has a slightly different form factor so it might be worth checking out in person, but I went from skeptical to really appreciating the design.

Any Kobo ereader

Much better for loading your own content than kindle as it supports more formats natively. You can also load alternative ereader applications to it via niclemenu opening it up further.

They also natively support borrowing books from your library through overdrive.

ONYX BOOX makes several different models, including some with color screens and most with reading light built in. They also have fantastic battery life. I have a Poke 2 which has lasted a month of active reading before needing to be charged.

These are basically Android tablets, so data transfer is not locked down. You can put whatever you want on it.

I know you said you don't care about apps, but on these you can run basically any Android app (within limits of e-ink screen functionality), including useful stuff like Obsidian for notes and Syncthing for syncing files across devices.

In hindsight, that's what I'd go for. You could have audiobookshelf, tachiyomi or any of your favorite android reading apps. But the Kindle does what it does well, and for relatively cheap. Maybe in 5 years I'll reconsider, if my kindle ever dies!
I just got a Boox Page to replace an early generation Kindle Paperwhite. Being able to load up android apps makes it much more useful (I find it to be great for cooking from recipes I store in Paprika) plus more flexibility with the eReader app you use.

I’ve had 3 ereaders and the only real changes I noticed between my oldest and newest is water resistance, lighting, and barely noticeable resolution improvements. Upgrading won’t fundamentally change your reading experience.

I personally don't think e-readers have improved drastically. I also have a 3rd gen Kindle keyboard and recently went through updating all the unlocks and screensavers I set up more than a decade ago.

While I was going through that it got me started thinking about them again. The only thing I think would be a big improvement is a light and more storage. I would also like to read manga on it. For those reasons I'm currently considering buying a Barnes and Noble Nook Glowlight 4 (or Plus) since it runs Android and you can install custom apps (like Tachiyomi for manga reading).

If your current Kindle is still doing the job and you don't find it lacking, I don't think you're missing out by not upgrading.

The newest Paperwhite from Amazon has a few things to recommend it.

The light is built in, and is still not technically a backlight.

It can produce warmer light to help you sleep and keep the pages more sepia toned.

It’s water resistant.

And it finally has USB-C.

I like to load up my own font choices and I can send whatever book I want to it, but if I own the audible and kindle version those sync between one another. So I can listen in the car and read before bed and won’t lose my place.

I really love mine. YMMV.

Whaa...sync audio and the digital word?
- Yeah, it’s really cool! Audible picks up where you left off on the Kindle, and the Kindle will ask if you want to jump to where you left off on Audible. I’ve only run into one series where it didn’t work.
  
  (Also buying the Audible narration for a Kindle book you already own is way cheaper than buying the Audible narration outright. Sometimes the Kindle + Audible is cheaper than the Audible.)
Whaa...sync audio and the digital word?

I've been seeing a lot of good stuff about PocketBook. They support all major ebook formats. They're also good if you're privacy focused, though it comes at a price. Their privacy policy makes them the only brand I'd ever consider. Here's a few links depending on your needs and budget:

Basic Lux 4 has a 6" monochrome display. This is their cheapest option.
InkPad lite is their biggest display at 9.7". An all-around solid, affordable option from them.
Era is a 7" monochrome display. I'm drawn to this one due to the functional bezel.
InkPad color 2 is a 7.8" color display. One of their most expensive options.

I already mentioned this elsewhere, but if you're concerned about privacy, you might consider Kobo as well; you can use their readers without having a Kobo account at all. That removes the store of course, but as long as you're ok with side loading your books, you're good.
If I had known about PocketBook I might not have bothered with my Kobo. But my Kobo Elipsa is nice.
- What about it is more convincing?
Hoping to hear feedback from anyone who has owned one
TIL: There are couloured ink displays now. Not sure if this is really necessary for my purposes though.
- Might be nice for comics if you're into that.

I've got a Meebook M6 that I'm very happy with. Its basically an e-Ink Android tablet with and SD slot and Google Play, so you can load the Kindle app or whatever you want if you've got that stuff. Most importantly, I use the Moon+ Reader app and load .epub/.cbz/etc formats plus it does an awesome job of reformatting .txt/.pdf/.lit. Bonus for me: Moon+ also supports custom fonts, so I can use Dyslexie.

The kindle paper white is nice but it’s Amazon. I have one that I keep in airplane mode and load books over usb.

I used a kindle keyboard for a long time and I think the new front lit ones are a big improvement, especially in low light (I use a kindle oasis.) I can't speak for sync improvements though since I keep it completely offline and transfer everything though calibre.

Wow that's an amazing run you've given that kindle! I used to have that one as well, years ago.

If you are sticking in the Amazon/Kindle world, then a new Kindle (whether you go with the base Kindle or the Paperwhite) you'll have a much faster experience for sure. The newer eink readers now can do several page turns without any real visual flash of the refresh. It's a small thing but very nice.

The Paperwhite has a backlight which is helpful for night reading. You'll also get dark mode in the new software (not sure if your keyboard kindle has stayed up to date with software updates or not). Also there's waterproofing.

The base kindle just loses the light and the waterproofing.

If you're not going to stick with Amazon, take a look at the Kobo readers. They're a great alternative to people not stuck in the Amazon world.

I’ve got a kindle keyboard 3 and want to know if there’s anything better out now

Better in what way?

KK3 + calibre is hard to beat. I own a paperwhite and some kind of nook (thrift store pickup for $10) and prefer the K3 experience the most. I typically end up using the paperwhite more because I am often in low-light conditions.

When these die my perfect ereader would be

eInk
android-based so I could run my preferred reader (FB Reader)
google play supported so I could choose my own browser

Kinda unrelated: has anyone used an e-ink Android reader? I want to use one for comic books with Tachiyomi. Is that a good idea?

Yeah, I've got the Boox Leaf2. It's fantastic. I haven't tried larger format Western comics, but manga is great in Tachiyomi. You might want something bigger that 6" for large comics. It's also great for pirating books straight on the device lol

The catch is that it's quite a bit more expensive than a Kobo or a Kindle, and battery life while good isn't as great. I found it was an well worth the upgrade.

I have a kindle paper white 6th gen and my first case expired a few years back, you can still get great cases for it with hard plastic instead of rubber, which is why my first case died :(

I would probably shop around for non-amazon stuff if it ever dies but it is the absolute minimum standard I would accept.