Nothing against Matrix, I haven't looked into it much, but have you looked into Keet.io? It's very simple, and I use it almost daily. Every device in the room is effectively a server, and the room mods are generally expected to run a device with it 24/7 to ensure everyone has connectivity, but it's not strictly required especially the more users your room has in it. It is lacking channel features from Discord, but more rooms can be created, of course.