If there's an expectation that you might merge a whole stack, I think you're aiming for the wrong thing. And it seems like landing the whole stack is the biggest push GitHub are making here. Part of the point is independently reviewable and independently mergable. If you're going to merge in one go then just put everything in one PR and review commit by commit in that PR. The commit by commit review flow used to suck, but since ~2021 it has been fine.
I feel like GitHub have built what users (who haven't used a true stacking system) asked for, not what they actually need to change their workflow for the better.
A great implementation would be asking hard questions like: what's the role of a single commit? Should PRs be single commit only? What's the real benefit here? How do you encourage smaller units of review (because it doesn't look like this does).