Why do you use Fast cast for Fire I? It makes no difference as you're still held back by GCD. Even if you weren't held back by GCD, using it before Thunder II and then Fire III just seems pointless and a waste of MP. Even with a Firestarter Proc saved for the Fire III after Thunder II, you'll be using it with no Astral Fire buff at all.
As for OP, what I do is Blizz III > Swiftcast > Thunder III > Fire III > Raging Strike > Fire I spam > and go into my rotation, which is the same but without Swiftcast and Raging Strike and replacing Thunder III with Thunder II. I'll use Thunder III again whenever Swiftcast is available, unless I'm in Garuda EM and the plumes group up nicely for a good old Flare.
I'll start casting Blizz III just as the tank is about to pull, too. There'll be no use using Swiftcast on anything other than Thunder III in this rotation as Blizz III reduces the cast time of Fire III below the GCD.
I never use Int Potions as an opener, though. Keep your Int Potions for strict DPS checks since you're likely to only get 1-2 uses per run.