Yeah numbers changes can only fix so much (Which don't get me wrong, BLM certainly needs some number changes)
Fire Phase is waaaay to rigid thanks to Flare Star. Numbers wouldn't exactly fix that.

Keeping the Astral gauge while under UI, I guess is one way to fix that, though another way thats been floating in my head is to change how the Astral guage gets filled + how Flare Star works
Example: Every Fire move fills the Astral Guage. Despair/Flare turn into Flare Star when at 0 MP. (This could be Ether when Astral gauge is full, or maybe the Astral gauge affects Flare Star's potency)
Ether way the Fire Phase can be shortened if the player wants, the latter might have some utility for a non standard rotation if the potency is high enough with short Fire phase, or I could imagine using Flare in a single target setting if the additional Astral Souls will make up the pot difference. Something like: (Manafont -> Fire IV -> Fire IV -> Triple cast, Flare -> Despair -> Fire Star).

The Fire phase is not as rigid and long it would probably help being ready to cast Thunder anytime when needed. Though Thunderhead should probably still be 40 sec