Me too. When they removed the option to download books I liberated everything I had ever bought, moved to Kavita+koreader and will never buy a kindle book again.
I jailbroke both kindles. And use koreader on them which now supports progress sync with Kavita which is amazing! So I don't really lose functionality.
Calibre loading books over wifi using KOreader made the jailbreak process worth it for me. My next will be a kobo or whatever else can run KOreader without hassle.
Yeah I'm not saying to buy a kindle to use KOReader. I just had the kindles and didn't really want to get rid of them, it saved me having to buy new hardware.
Kavita was mainly designed as a comic reader, but it has recently invested a lot of time into epub quality of life features, I can recommend it. It already supported epub but lately it feels more like a first class citizen.
Since the last major jailbreak for Kindle devices was released, I've been using Koreader as well on my Scribe. I have progress syncing setup with Hardcover (Goodreads alternative) instead, however. Only downside is their recommendations don't seem to be well geared to my interests at this point, but hopefully that'll change in the future as more use the service.
I jailbroke both kindles. And use koreader on them which now supports progress sync with Kavita which is amazing! So I don't really lose functionality.