However, pmOS is pretty unique in their quest to mainline other, closed, Android smartphones by making shitty code vendors put out good enough to be merged and by making their own drivers and shims to get hardware to work.

I don't see why you'd need a Pinephone to work on pmOS, unless you want to develop a pmOS application or feature for an ARM device that has all the features up and running. It's more lightweight than some of its competition, for sure, but honestly it's not really better or worse in terms of hackability.