How to use approval subtask to trigger main task changes?

While this might be of value, it would let you see a difference between an Approval task that’s marked as Approved vs. Rejected; both will satisfy the “All subtasks complete” trigger.

I think the only current way to manage this is to not use Approval subtasks but make your own single-select custom field for Approvals/Phases and rules around it, because then you don’t get into the situation of parent/subtask limitations with rules. (You can still add subtasks in actions if helpful, though, of course.)