MongoD hung, so earlyoom is now installed, running, enabled and hopefully set up correctly (with --prefer '(mongod|lynxchan)' and -m 2).
This means sending a SIGTERM at less then 2% memory, and a SIGKILL at less than 1%, which when combined with the Restart=Always on systemd should hopefully cleanly restart if it gets too high, just before hanging. Automatically instead of my manually doing this.
If it starts to get stuck on a loop, the only option beyond minor optimizations would be a server upgrade.