There are really good talks on Reddit about changing out blizzard 3 with transpose then using thunder 2 instead of 3 to fill in the umbral phase. I think the two rotations are something like:

Thunder 2 > Fire 3 > Fire til OOM (for the whole first phase using procs for Thunder 3 / Fire 3 ) transpose > Thunder 2 > Fire 3

I had trouble with transpose, and had to end up looking too much at MP. I end up doing something like:

Thunder 2 > Fire 3 > Fire til OOM ( procs for Thunder 3 / Fire 3 ) > Blizzard 3 (Since it casts a lot faster due to Astral III) > Thunder 2 > By this time if I time the Fire 3 with the next tick of Umbral III then I'm at full mana and can continue the rotation.

I guess you could use Blizzard 1 instead, I'm not positive if the Umbral I vs III has any difference on regen. It didn't seem to matter much when I was playing before, but I haven't really looked into it. If that's the case then you could fire 1 for longer, then blizzard 1 > thunder 2 > fire 3.