I created an account to say thanks for this awesome thread, especially for all the research hollowpoint has done. I helped me a lot in my own search. I finally ended up using FB reader's network to sync my books and annotations. I'm using an Onyx Boox device and FBreader's latest beta plays really nice with it. The only downside is they don't have a very good desktop app yet, but apparently they are working on it again:
https://fbreader.org/content/end-2021-update
In case anyone's interested, I've described my search here
https://www.reddit.com/r/eink/commen..._syncing_epub/