I think you could just make Flare Star activate as soon as your MP drops to 0 in fire phase.

To make it a bit more restrictive, you could also have every blue umbral heart spent by F4 or Flare turn into an orange astral heart instead, and Flare Star spends those three hearts upon being cast. Or flare star could just cost three astral hearts and otherwise not care about your MP at all, so you could put it anywhere in the back half of your astral cycle you please but still can't skip B4.