So then my next problem: I can't seem to get my configs to stick.
Selected 'Edit config' and replaced the default contents with something I'd already prepared.
Selected 'Apply Config' (only working button in the admin ui FWIW).
Change to routes, change back to ACLs/Config and the code I just added is still gone. Guess arcoast's fix doesn't cover that issue.
Edit config
Paste in settings again
Selected apply config
*Selected Save Config*
"Saved ACL Configuration" appears at the bottom of my screen.
Change #headscale-admin routes, change back to ACLs/Config and the code is still there.
OMFG. I just... I ... aaaargh.