XMPP or Matrix. I’m on Matrix only because I have my family there and I was there before I knew of XMPP and at this point I can’t turn that boat.
Signal was/is (idk if they still are) into crypto, they don’t let you run your own server or client, and they have a proprietary shim in place to combat spam (or so they say, it can’t be audited because it’s proprietary).