This is exciting! A new way to increase our single target on shorter fights.... How do you handle using your cooldowns off the bat? Surely you wouldn't want to use phoenix's outside of combust, would you, maybe one? So use all fireblasts, then start chaining fightball + pyros until 90%?
PF actually gets almost no benefit from combustion at all, it's totally inconsequential if you use PF outside of combustion if you are able to generate max Pyros during combustion.
ideally now with 14 second combustion you wanna open with fireball, MI, FB, fireball, pyro, pyro, PF, pyro, fireball, FB, pyro, and from here chain FB pyro using FB every time you are about to cap at 3. then as soon as you get to 90% pop combustion.
Obviously you wanna use bracer proc as soon as it's up and if you have meteor it's probably better to pop it right after mi.