Not many seconds passed after I wrote the post above before I realized it’s still too limiting.
One thing it still doesn’t solve is this: Detect subdomain via router, or globally in pages or Cells
Another thing is, what if I want to pass props to my layouts? Can’t do that if they’re automagically wrapped around the pages.
Looks more and more like the easiest option would just be to go with React Router 
Either as-is, or build something on-top of it.