

In my experience matrix is a total mess with plenty of bugs, also uses quite a lot server resources. xmpp is lighter, easier to set up and works flawlessly, and seem also easier to proxy or do stuff like that with it. Calls are kinda messy to set up tho.

I have tried every possible set up, I’ve got to say that the best one so far for me is sychthing + Koreader: dead simple, no databases, synchs annotations and highlights as well, books are simply epub files + metadata.
Aka no dealing with exports, database corruption or stuff like that.