I found a fairly low-friction way to add a custom online services subsystem into UE4/5: https://github.com/RyroNZ/UE4MasterServer

You can replace the python with anything that lives across JSON. I had no issues getting this to compile & load in UE5.

Once you have this in place, you could grab one of their arena shooter MP prototypes and focus almost entirely on the sessions, servers, matchmaking, accounts, logging, fleet management, etc.