FYI there are alternative matrix servers to Synapse. I’m playing around with Tuwunel, it’s a single binary and you’re done.

It uses rocksdb so you just give it a directory and it puts everything there,. ie no need to spin up a db. And there is a config option to disable federation if that’s something you don’t want.

Could it scale to thousands of concurrent users? Probably not but it seems to be the simplest chat app to self host I’ve found.