TLDR; Thirty years after the advent of the first instant messaging services, we still haven't reached the stage where instant messaging platforms can freely communicate with each other, as is the case with email. In 1999, the Jabber/XMPP protocol was created and standardized for this purpose by the Internet Engineering Task Force (IETF). Since then,
For messaging is recently tried SimpleX with a group of people I know.
It’s very nice, it’s open source, does not require a phone number, is e2e encrypted, has a very nice UX and UI and is decentralized.
By curiosity I try to setup my own server and it was up and running in 20 min. I was really impressed by how easy it was.
I have not try it for very long yet but so far I find it better than Signal.