This is not an anti-Kindle rant. I have purchased (rented?) several Kindle titles myself.
However, YSK that you are only licensing access to the book from Amazon, you don’t own it like a physical book.
There have been cases where Amazon deletes a title from all devices. (Ironically, one version of “1984” was one such title).
https://www.nytimes.com/2009/07/18/technology/companies/18amazon.html
There have also been cases where a customer violated Amazon’s terms of service and lost access to all of their Kindle e-books. Amazon has all the power in this relationship. They can and do change the rules on us lowly peasants from time to time.
Here are the terms of use:
https://www.amazon.com/gp/help/customer/display.html?nodeId=201014950
Note, there are indeed ways to download your books and import them into something like Calibre (and remove the DRM from the books). If you do some web searches (and/or search YouTube) you can probably figure it out.
The only issues I ever had were around authors having a bunch of books that weren’t released or were in different languages, that was solved by narrowing the profiles for what readarr finds which was a 2 minute task
Years of ongoing issues with their metadata server bricking its ability to search for content. It wasn’t an issue with your setup, it’s an issue with Readarr itself. They always fix it, but it’s kind of a joke how many times they’ve had the same problem over the years.