Whilst trying to make sure all my thoughts are gathered here in suggestions, I'd like to add one thing that I've found users struggle with, and that's adding a new page. As much as I try and communicate how to use master pages, it often ends up becoming messy and pages with various master pages.
I think to solve the issue, It'd be really nice to be able to set a master page for a top-level page, that the subpages either default too, or is just automatically set to reduce the number of inputs so users just have to worry about a page name, a slug being automatically created. I think this would drastically reduce the amount of confusion around how to do that, especially for larger sites with quite a few master pages.
Does that make sense? If I can help with any use cases or as always, if you have any questions, please don't hesitate to ask!