Respectfully I disagree with lack of Astral Fire III being the real issue.
The biggest issue with Fire II is that it's meant to be the spammer spell for AoE fire phase, but its cost is twice that of its single target sibling Fire I. This means you can only afford to cast it twice before you have to switch back to ice. That's the root of its biggest losses.
If it's meant to be the spammer spell for AoE, make it cost the same as the spammer spell for single target and give it enough damage to make it worth casting five times once you average in the Freeze and Fire III recovery. If the base single target rotation from 35 to 49 is meant to be
Blizzard III > Thunder > Fire III > Fire > Fire > Fire > Fire > Fire
then the base AoE rotation should be
Freeze > Thunder II > Fire III > Fire II > Fire II > Fire II > Fire II > Fire II
and the MP costs and potencies should be adjusted to support this.