Hi gatomon
As always, things depend on your gear and I can only tell you things I experienced with my gear, so ultimately you'll have to sim yourself to find what's best for you.
That being said, here's what I think might be some problems you're having:
- During ToS, fire mage didn't require crit, due to the fact, that thanks to the scorch belt + firestarter, critting was a non issue during >90% and <30% boss hp. Adding to that the t20 2p bonus also devalued crit by giving you guaranteed crits. Now you're no longer using the belt and you lost the t20 2p, so I suspect crit should sim quite a lot higher for you now than the other stats.
- You used Tarnished Sentinel Medallion on your first combustion and then as soon as it came off cooldown. This didn't line up with your following combustions and you didn't get the full use of your third usage due to the fight ending. So delaying the second use until your next combustion would've increased your damage from the trinket. See Here
- Using the gloves to reduce the cooldown of your Combustion can be strong, especially since t21 4p increases your damage during combustion by 10%. However, your third and fourth combustion were not as strong, because you had to use scorch during it.
See Here
- You're using Incanter's Flow in a Kindling/Glove build, which I suspect to be sub-par. Between your combustions your damage is very low, so you don't gain a lot from IF and during the combustions IF doesn't push you as much as RoP would.
- You're not using the BL ring in heroic. The duration of the fight is only 4:30, which means with the ring your BL up-time would be almost 50%. Your raid used BL at the end of the fight however, so this might not be an option. Using BL ring on heroic encounters is very strong, since they're usually very short. Not being able to exploit that will make it very hard to parse high.
- You don't use Fire Blast during your opener. When you use Firestarter you can kinda play as if you're in a combustion, since you're guaranteed to crit. Just make sure you don't go all out so you don't have anything to use during the combustion once the boss drops below 90%
- Your combustions are very messy. You munch proccs and start combustions while not having an instant Pyro ready. I suggest going to a training dummy and practicing how you enter a combustion. Remember that you can use combustion while casting Fireball, or even better while casting a bracer procc. I think at one point you even had to soak during your combustion, so you were not in range of the boss. Look at your bossmod timer whenever you prepare for a combustion. If there's a mechanic coming up that you absolutely have to play, maybe you should delay the combustion until after said mechanic. (especially important when using RoP)
So how can you fix this?
I think you should move away from using the gloves and convince your raid to use BL on pull so you can use the ring. (If that's not an option maybe you could try the belt.)
If you can increase your crit ratings, you should be able to reduce your combustion cooldown almost as much as you're doing now with the gloves. I'm currently bouncing between 60% and 70% crit depending on trinkets and legendary configuration.
Also try out using RoP during your combustions to exploit the low cooldown on it.
Fix your opener. I'm using the following opener (I'm not sure if it's 100% optimal):
Pot > Pre-cast Pyro > PF > Pyro > Fire Blast > Pyro > Fireball > Fire Blast > Pyro > Fireball > Pyro > Fireball > Pyro > etc. (don't let Fire Blast stacks reach 3)
Depending on your haste values you should start pre-casting Pyro at around 4 seconds left until pull.
But again, I have to stress that you should sim yourself to see where you're standing at any given time. In times of Legendaries, WF & TF it's very hard to give advice that holds across the board.
