Question

Any block that can fill this space? Want to be sure I'm not missing anything

On left side it is a Framed Slope and on the right side it is a Framed Extended Slope Panel

Possible solution, a way to lock the center of the Framed Collapsible Block and then move the corner to be able to fill the space in the screenshots above

There is no existing shape to fill that spot, but they've been planned for quite a while already, see #226.
Adapting the collapsible cube for this is not a good option, that block is already pretty complicated under the hood and would need to become significantly more complicated to accommodate a behaviour like this.