The main difference between SimpleX/Session and XMPP/Matrix is that:

SimpleX/Session hide metadata even through you're using other people's servers.

While as XMPP/Matrix don't hide the metadata at all, but expect & empower the end user to run their own server.

So something like self-hosted XMPP, with a Tor Onion, is a higher level of privacy than trusting random SimpleX/Session servers, but also a higher barrier to entry, as most are not willing to read docs and pay for a VPS.

Side note: You can configure an XMPP client like Gajim to force Tor, even if it's not system-wide.

SimpleX does have a self-host, but if you just use your own server, then it labels/identities these accounts as you. So you'd have to have a popular server, and even then the other person in your conversations picks the other server.

Ok, let's pause here and resume tomorrow.