In another topic thread, I proposed an alternate workflow for this situation: Mark subtasks complete when completing parent task. In short, it’s an attempt to solve the same problem, but rather than forcing a sequence, it empowers the user to check off all the subtasks at once (if desired).
In the first reply in that thread, I added some suggestions to improve the behavior described in my original post — namely, a confirmation to make sure users are aware that they are completing multiple subtasks at once. Would be interested in any feedback, yay or nay, on the approach described in that thread (please make those comments there, not here).