Anyone know of a fork of signal that disables this keeps the SMS feature?
Anyway, good luck getting Signal to federate with you until you have enough of a user base that they're losing users to your backend. They have zero reason to want to do this, and it introduces some privacy issues (what if your server doesn't respect deletions, etc?).
XMPP is just SMTP and some protocol addons on top. SMTP is just FTP with RFC 469 and others. Etc.
I haven't used but it is supposed to be a hardened version of signal.
Not that it supports SMS, but it _is_ a fork that uses Signal's servers.