I went through this migration last year. A few things that helped:

Calibre is the escape hatch. Converts everything to EPUB. Even if you don't use it day-to-day, it's the best tool for getting your library out of Amazon's format.

Public domain catalogs are huge now. Standard Ebooks, Internet Archive, Gutenberg - tens of thousands of well-formatted free EPUBs. Most people don't realize how much is out there.

For actually reading on macOS/iOS, I ended up on BookShelves (https://getbookshelves.app) after trying a few options. Native app, reads EPUB and comics, has Calibre wireless sync, and browses those public domain catalogs directly. Books are just files on your device - no account, no cloud lock-in.

Honestly the hardest part was realizing how much of my library I'd been renting rather than owning.