As long as you don't need audiobooks on it, you can essentially erase your wifi credentials, turn off wifi, and manage your ebooks through USB and something like Calibre.
The Kindle OS will continue to collect and store all sorts of telemetry, and will upload it later if you ever reconnect the device to the internet. Just something to consider.
Normally, I would just suggest avoiding Kindles but, like me, you already have the device so might as well use it.
FWIW I got a helluva deal second hand, so I didn't give Amazon money lol. I only used it a little bit with manually loaded files, so if I use it again I may just do this.
I’ve never tried sending ebooks to the Kindle Oasis via USB…that’s an option? Never even tried connecting it to my laptop. The Kindle and Calibre talk to each other pretty easily that way?
I have a Paperwhite myself, but from what I've read the Oasis works just as well via USB and is equally compatible with Calibre. The experience is pretty much plug-n-play after the initial setup wizard.
A jailbreak method called Language break was discovered relatively recently, so even if you're on a more recent firmware version, you could jailbreak and install KOReader on it.
If it's an older version of firmware, there are also other method of jailbreaking afaik.