Without doing an exhaustive amount of math on it, all the flourishes provide approximately the same boost to WS damage (~30%).
Using any of them sacrifices time that you could have used to gain more TP or finishing moves that you could have used to gain more TP. In the case of a 15 TP WS return (85 TP to the next WS), losing 40 TP for Striking is about half a WS. Would you rather have a 30% increase in WS damage or 50% of another WS?
So they aren't worth using with it. Which one is the best is going to depend on the target's defense, but Ternary is more competitive on this WS that it has ever been (because it is low fTP). Still, Ternary is 57 TP. WS + 2 Rounds + 3FM Reverse Flourish is another WS.
Practically, do you do self darkness often? I probably only find it useful on soloing Limbus, but not quite on anywhere else. And given the huge base damage on Exenterator (and hence high closing WS damage), the Fragmentation created by Evis > Exen probably isn't lagging much behind than darkness crated by PK>Evis.