I feel like all DRG's GCD rotation needs to make it interesting is to add a dynamic finisher without a positional requirement that you could use at any point instead of F&C/WT or after using one or both in their appropriate sequences, with higher potency scaling than they have to maintain average DPS, but not high enough to make the positionals not matter. I don't feel that F&C and WT on their own are that interesting either. However, I can see how to make the combo length and order matter and be more interesting, and offer multiple breakpoints and rotations based on skill speed and fight dynamics, which is worth keeping the current combos around for. And it only takes one button to accomplish this. Using mechanics that they've already introduced into DRG's current kit.
I don't feel that we need to start their combos from a single point so much as we need to have Heavy Thrust merged into Impulse Drive and that finisher to be added. Sure, SE could pair it down a little more and merge ID and HT with True Thrust, but it doesn't really make sense to allow their filler combo to apply their damage buff. Might as well bake it into their potencies instead at that point, basically. It would likely shift to Disembowel or Chaos Thrust in that case, which makes even less sense given the names used for those skills, without adding much more to them either.