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.
That's stupid. Nothing stops you from just installing regular Ubuntu if you love snaps so much.