This is very much my take. As long as the general rule is a lack of long PRs, I think we get into a good place. Blueskying, scaffolding, all sorts of things reasonably end up in long PRs.
But, it becomes incumbent on the author to write a guide for reviewing the request, to call the reviewer's attention to areas of interest, perhaps even to outline decisions made.
> on the author to write a guide for reviewing the request
I'm not saying that doesn't work, but writing a guide means the author is now also doing all the planning too.
A successful "guide" then becomes more about convincing the reviewer. The outcome is either a lot of friction, or the reviewer is just going through the motions and trust is eroding.