Installing Synapse is very easy actually with Pip, however as this OP showed that using your own identity server is also necessity. FOr that you want https://github.com/kamax-matrix/mxisd
But this is what I've been pointing at; to self host XMPP with multiple domains and per domain settings, I need prosody, nothing else. Not identity server, no video servers, etc. This is the based of my problems.