I've considered doing a sweep to download all of my kindle books and de-DRM them so that I have an archive, but this is a tortuous process if your library has over a thousand titles, as mine does.
The process is really not bad at all if you use the desktop Kindle app to download your library before importing to Calibre. Each step is fully automated with the only manual parts being setting it up and doing each step in sequence for the whole library but not each individual book.
So no, I wouldn't say you missed anything obvious, which is a feature not a bug as far as Amazon is concerned.
The workarounds mostly involve getting Amazon to give you the book in an older format, but then you lose the typography improvements that KFX gets you.
Apple's DRM format (Fairplay?) has never been cracked but I believe Adobe's has. Buying from the Google store of Kobo store is probably the best bet.
The book DRM problem requires a legislative solution.
I second buying from Google Play. Outside of a period of time last year where they had a bug that prevented exporting many titles in their catalog (some error in their backend service), I have never had an issue with purchases from there. I will happily continue getting my books from sources that allow true ownership after purchase regardless of any touted benefits Amazon adds to future DRM schemes, just need the words on the page.
For me, the kerning, hyphenation, and spacing improvements in KFX are pretty big. I also like that I can choose justified or ragged right.
https://www.reddit.com/r/kindle/comments/viqxjj/heres_the_fo...
Regardless, my point was that: A - most of the books already in the library were likely enjoyed/acquired without knowledge of hypothetical improvements from Amazon rolling out a new ebook format and DRM scheme. And B - even if there is some magic that Amazon had to include in KFX to support the improvements you listed and can't be reproduced without them; I personally would not consider those or most any improvements to be worth losing ownership of books that I purchase. The most valuable part of an ebook is the text and ownership of a copy of that is what I'm paying for. It is fairly easy for me to be principaled on only buying ebooks that I know I can own a copy of due to the diverse distribution that exists for most titles. Even when I had an Oasis, I didn't purchase anything through Amazon and loaded all my books over USB.
Seems like that's how it should work, but it doesn't. Maybe that's by design or maybe it's fallout from poor choices Amazon made earlier in Kindle history. I really don't know.