Hi - I think if you look at the 'basic' version of Planner as a task management solution, and the new Planner 'premium' as a project management solution, that should help differentiate between the two. Which is why when you move from one to the other you see gaps.
Premium may NOT be a fit for doing task management, esp given it has a scheduling engine behind it that drives all activities in the system (versus independent tasks with no ability to link). Alot of this may not be well communicated in the app or on the public Microsoft website, but some search into YouTube and Google and you can find a wealth of data on each one. It's all done with purpose by Microsoft...
I know it doesn't ease the current pain either of you have noted, but each tool has a different purpose, and different matching functionality. Should most functionality carry over from one to the other - absolutely. Does it - no, because things like 'recurring tasks' when put into the context of a project schedule (versus just a 'set of tasks' in a Plan) means something different.