What makes Flare Star feel "stuck" at this point is that the requirements to cast it are possible but very easy to entirely fail, and because of that Flare Star's potency needs to be limited to make sure that failure doesn't result in a devastating loss of damage output.

The easy solution to that is to just mitigate the possibility for such extreme failure. One easy way to do that would be to just let the Astral Gauge persist through Umbral Ice. Now instead of losing ALL of your Flare Star progress because you couldn't get that last Fire IV off during an Astral Fire phase, you only lose 1/6 of your Flare Star progress. By making imperfection less punishing and allowing players to keep their progress, you can now send Flare Star's potency to the moon without causing a drastic disparity between sub-optimal play and optimal play.

And as a bonus, now less skilled players get to dependably cast Flare Star, which they'll enjoy because it's their big boom...they'll just be casting it less often than their more skilled peers. And hey, now your Astral Fire phase doesn't have two finishers, because Flare Star can be held over to the beginning of your Fire Phase; as a BLM, I like having more flexibility.

(And as a side-note, Flare Star shouldn't be able to be assigned to the hotbar; Fire IV should become Flare Star when your Astral Gauge is full, since there's no sensible reason to cast Fire IV at that time.)