I politely disagree with you, @Derrek!
It’s fine to create a Master Page that’s based on another Master Page. The crucial thing is that any frames that are pulled in from the original Master Page will remain linked for evermore. The chances are that, for instance, the music frame and the text frame containing the page number token can be left exactly as they are.
If you need the running header text frame to be different, delete that frame - just that frame - and recreate it. That way, just that text frame will not be linked back to the original master page.
(This is documented in the Note at the top of this page: Customizing master pages)