The Feature.

The title essentially says the request and I'm not exactly sure how to explain it in more detail, but it's something like a corner slope but slab, I also figured that this issue could be completely resolved by allowing the collapsible box to deform in two directions at once, though I assume since it hasn't been done already, it would either be very difficult to do, or would cause more problems than it solves. I'm not sure if it's relevant but I'm using the latest update for the mod on 1.18.2

If it hasn't been mentioned already, the proposed "Framed corner slope slab / panel"

Someone else had previously suggested something similar, I'll have to check to what degree that overlaps with your suggestion.
Though, I can't guarantee you whether this is viable, I'll have to look into that when I have a chance.

Expanding the collapsible cube is however not possible, that implementation is already fairly complex.