For ebooks, the hurtle is really storing them on your own system. Most open source readers can read Amazon's .azw and .azw3. I like to keep everything in epub on my proton drive, but I buy (and sometimes don't buy) them from different places. I went to my local library to come up with a archival method that works. I use an open source ereader app on my phone and laptop, and use 'Send to Kindle' to get books on an old Kindle from 2014, deleting them when I'm finished reading.

By breaking up where I get my books from, and decoupling it from storage and reading method, I'm able to be a lot more flexible and take advantage of deals/exclusives on different platforms. I also feel a little more private. The downside is paying for storage, and an extra step to download books from the central location when I want to read them.