I’ll defer to the coaches on this one.
What you describe is similar to the practical limitations I also face.
I’m only about two months into experimenting with power, but I see enough correlation that I feel I can rely on power to substitute for Pace on hill workouts. However, some of the hill sprints are so short that even power doesn’t seem to be picked up quick enough for real-time monitoring, so I just go all out. Power is be more useful for post workout evaluation.
If I am not mistaken the objective of hill workouts is to develop maximum strength/power. In that regard power seems to be the best metric.
I’m just learning, so I leave it to others to correct me. I have no pride so feel free to do so…