XMPP maybe. Matrix is a bloated protocol which costs a lot more to host.
kpw
There's an IETF internet standard for federated messaging called XMPP. Just be compatible with the standard. It also allows for extensions if you offer more than the core spec.
I honestly don't know what you mean by that. I use Arch btw.
I can't read Chinese, only Japanese, but I think it's asking for a name and the number of a personal ID which should have 18 digits.
Sway works well for me, what's wrong with it?
Firefox + uBlockOrigin
Wikimedia ist ein typisches Beispiel, wie eine Organisation sich so lange ausbreitet, bis das ganze Budget verbraucht ist. Die Hosting Kosten sind für lange Zeit gedeckt und diejenigen, die die Artikel schreiben, machen das ehrenamtlich. Die Wikipedia ist eine wirklich tolle Sache, aber es gibt andere Organisationen, die Spenden mehr verdient hätten.
Conversations from F-Droid is pretty solid.
Prosody and Openfire are servers while end-to-end encryption happens on the client side (that's why it's called end-to-end). It would be kind of strange if a server implementation talks about E2EE. The OMEMO protocol only needs server features which are widely implemented. Maybe there is an ancient XMPP server implementation out there that doesn't support it, but you will be fine with Prosody, Snikket, ejabberd or anything else really.
If you need to convince your friends to use some app it might as well be XMPP compatible instead of another walled garden. If you can get your friends on board, you win, even if nobody else uses it.