For mono Unity stuff, I’ll use dnSpy (by way of the dnSpyEx fork) to decompile the game, test changes by editing the DLL directly with it, then port everything over to BepInEx in some form or another.

I’ve used XMPP for this before, using ejabberd as the server and Conversations/Monal as the client, it worked pretty well, altho ejabberd can be a bit of a handful to get running initially.