Interesting... After 72 you would have just enough mp to hardcast a thunder 3 during your fire phase if you needed to. That hadn't occurred to me before. And after 74, when sharpcast is pretty much guaranteed for every fire phase you can hold on to it for a few seconds...

I've now been playing around with just re-upping thunder as it is needed. and my fire phase looks like: Fire3/sharpcast > Fire4 x1-2 > Fire1 > Fire4 x1-3 > Fire3 > Fire4 until despair, which leaves me 3 different spots in the fire phase where I have enough time to hard cast thunder 3 if i need to as well as the standard ice phase cast if it's needed. It's a bit less cut-and-dry so you need to be a bit better at keeping track of the duration of thunder 3, but each window for thunder 3 could also be used for xenoglossy if you are about to time-out on a third stack.

This also gives huge windows to move out of mechanics if you need to without running much risk of losing AF/UI.