I have self-hosted Matrix for a couple of years. I eventually settled with xmpp/ejabberd/conversations. It is easier to maintain and less resource intensive, especially if you want to go the federation route.

This

And so, where is the problem?.. Your message is scanned. If it is illegitimate you can’t send it. What is the problem?

What I do is a local backup on a different disk with BorgBackup, then a copy of that local backup to a Pi at a friend’s place, with rsync.