If the goal is make Fire I, Blizzard I, and Scathe useful the BLM so that we can have a reason to keep them on our hotbars, I feel like it would be simpler to do the following:

Blizzard I
Deals ice damage with a potency of 180.
Additional Effect: Grants Umbral Ice or removes Astral Fire
Cast Time: 2.5s -> Instant

Fire I
Deals fire damage with a potency of 180.
Additional Effect: Grants Astral Fire or removes Umbral Ice
Additional Effect: 40% chance to grant Firestarter -> Grants Firestarter
Firestarter Effect: Next Fire III will cost no MP and have no cast time
Cast Time: 2.5s -> Instant

Scathe
Deals unaspected damage with a potency of 100.
Additional Effect: 20% chance potency will double. -> Potency increases as your MP decreases, reaching its maximum value of 400 when you have 30% MP or less.
Additional Effect: Becomes Xenoglossy when under the effect of Polyglot

The changes I'm suggesting are marked in bold. The ones for Fire I and Blizzard I would be made at level 70, and would only come into play when switching from Astral Fire to Umbral Ice (the Blizzard I changes) or vice versa (the Fire I changes).

The first Scathe effect change would be implemented at the start, and could eventually get a potency increase to 600 at level 70. The Xenoglossy Change would be added in at level 80 and would be a permanent change.

These changes would not only help justify keeping these buttons on our hotbars, but would also help players get used to the timing for Paradox. Having Scathe get more powerful the less MP we have would also encourage players to get used to the idea of having a "finisher" cast in our rotation early on.

As for the AoE rotation, the only thing that will help with that is to either a) give potency boosts to High Fire II and High Blizzard II, or b) give BLM a new mechanic that allows consecutive uses of High Fire II and High Blizzard II to boost the effectiveness of Flare and Freeze respectively.