There’s Librera FD, on F-droid. It’s the open source version of Librera on the google play store. I believe it strips out the google api/cloud stuff, but its db /reading status/etc are all easily synced via Nextcloud (which is what I do). The only real negative I’ve found with it is that it’s search for text in books is slow compared to more popular reading apps like Moon+ Reader.
Aegis is for Android, Ravio is an iOS app.
It is still possible if you have an old enough Kindle. I have a 1st gen paperwhite and can just connect it via USB to my PC and pull books off the kindle with Calibre and then remove the DRM.
Though if you’re wanting to remove DRM from Kindle Unlimited books, you’ll have to actually edit the DeDRM plugin code, as by default its set to not let it be run on KU books.
Have you tried adding it to Steam as a non-steam game and then forcing compatibility for it to proton experimental in Steam?
That’s stupid. Nothing stops you from just installing regular Ubuntu if you love snaps so much.