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.

Honestly I just keep mine on airplane mode. Calibre was always a much better management tool, and libgen isn’t nearly as creepy.

Check out koreader to put another OS on there. Might be what you’re looking for

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.