Would it be easy to Clone the Plan (you can do this directly in Planner), delete whatever you don't need?
Now, as for your question. Please understand, you said but shared no details. There are loads of examples of how to do this, so we want to help you, but am unable to know where to start without your flow, expressions etc.
Problem: I am running into trouble getting all of the task details copied over (labels, assignments, priority, description, checklist, etc.).
What I will say is, once I release my connector etc it will do it all for you with a simple action.
But please share your flow and share exactly what the issue is you are having and happy to help.
Until then I did a simple search on Bing and here are some very solid links to help you.
I will say that in some cases, you may (or absolutely do) need to use Graph API calls to copy data or create things, simply due to the limitations on the planner connector.
I hope this help and please let me know what specific actions I can help with
have a great day!!
