I'm pretty sure FotI is always worth using on ST regardless of whether or not you have RotI purely because of much damage it does, though it's preferable that you use it before popping Metamorphosis as that's time better spent building towards or casting Annihilation.
One question I had was if it is ever worth stacking haste until a certain breakpoint in order to squeeze another ability usage inside the Momentum window or if it is simply infeasible to achieve.
As it stands, I'm currently at 845~ ilvl with 44% crit, 7% haste, 8/4% versatility, and 12% (4%) mastery; with these stats, I'm really starting to feel the rotation slow down a lot outside of Metamorphosis due to sacrificing haste for versatility, to the point of sometimes losing the momentum buff before I can finish a cast if I make a movement error with Fel Rush or the like.
Another question I had was just how valuable mastery really is outside of a ST encounter. I believe one of the reasons versatility is weighted so heavily above mastery is Throw Glaive + Bloodlet in addition to Demon's Bite on single-target, but in a Mythic+ environment, would mastery be more desirable due to nearly all your AoE abilities (Fel Barrage, Eye Beam, Fel Rush, FotI) being chaos damage? I feel like in a dungeon I would want Crit > Mastery >= Vers > Haste, haste being so low because Havoc is themed around high burst AoE, but haste only serves to burn through your rotation faster rather than amplifying those huge cooldown's effectiveness.