Had a hard lock of a new computer I'm setting up. Instinctively, I did SysRq + REISUB as I've been thought, then continued troubleshooting. Figured that maybe not everyone knows about this rescue command :)

The Wikipedia page isn't great, here's a TLDR for forcing a clean shutdown, even if the computer locked up, as long as the kernel is still alive:

  - Hold the SysRq key and press this sequence of keys, with a short pause between each:
    - R = switch keyboard from "raw" mode (so X/Wayland can’t intercept)
    - E = send SIGTERM to most processes
    - I = send SIGKILL to most processes
    - S = sync disks (flush writes)
    - U = remount filesystems read-only
    - B = immediately reboot (no further shutdown steps)

Mnemonic I personally use, bet others have better ones: "Reboot Even If System Utterly Broken", REISUB!