Though it's possible that generating the STEP first is easier to do, and that the plan could be backporting the feature tree using another method / model would then enable editing.

Yes, it would seem post hoc feature tree requires the constraints that come from context in your head, but I could imagine that for most cases a "drafter's intuition" in AI may be sufficient, and you could build an interface to allow that to be mostly given up front and then through iterate post generation.

I could imagine the stepwise approach may allow AI training to be more constrained / efficient that trying to do the whole thing in one go.