That's neat, I'd never considered one could use udev reactively like this! Very nice.

Not nearly as elegant, but I keep running into problems where I want to run some command when X happens. I've been trying to lean into systemd, creating a service for whatever it is I want run (perhaps just a oneshot if it's a a one off command). Then have a separate program that detects for the desired condition (polling?) and runs systemd-notify to signal that it's healthy. Then you can just BindTo=, so that the desired command runs whenever the condition-detector thing goes green.