I think you might be interested in NUTs, Network UPS Tools. They provide a way to distribute power information about a ups across a network.

https://networkupstools.org/

Looks interesting, but my main obstacle right now is that the power switch is controlled via HomeKit - I could make my Mac an integral part of the setup (it's powered 24x7 after all), but that feels clunky. I think the way to go is to make a shortcut to gracefully shut down via Mac/iPhone. It's one tap, just requires some thinking to set up.