IMO the biggest attack vector there would be a Minecraft exploit like log4j, so the most important part to me would make sure the game server is properly sandboxed just in case. Start from a point of view of, the attacker breached Minecraft and has shell access to that user. What can they do from there? Ideally, nothing useful other than maybe running a crypto miner. Don’t reuse passwords obviously.
With systemd, I’d use the various Protect* directives like ProtectHome, ProtectSystem=full, or failing that, a container (Docker, Podman, LXC, manually, there’s options). Just a bare Alpine container with Java would be pretty ideal, as you can’t exploit sudo or some other SUID binaries if they don’t exist in the first place.
That said the WireGuard solution is ideal because it limits potential attackers to people you handed a key, so at least you’d know who breached you.
I’ve fogotten Minecraft servers online and really nothing happened whatsoever.