I got fed up with Amazon's locking me into their eco-system by using non-standards about a year and a half ago. I'd suggest buying a Kobo (Sage for bigger screen, Libra 2 otherwise) and converting everything over to EPUBs. You can then buy your books from anyplace that sells standard epubs (Kobo Store (will price match), Google Play Store, etc.. Check out the Kobo forums:
https://www.mobileread.com/forums/fo...play.php?f=223
And, if you decide to convert, here's a thread talking about some things to do:
https://www.mobileread.com/forums/sh...97#post4124197