Blocking is multi piece for a riser. The riser also needs to project 3/4 beyond the box, to be flush with the doors on the front.

Youtube for an example: How to Install Riser Molding...

A 3/4” riser piece will always be thicker than a 1/2” cabinet side. That is not how blocking for that is done. Yes, the riser has to extend past the cabinet box.

Just use a few pieces of tape to shim out your mounting block the desires distance

Very generally, try to avoid making two surfaces coplanar (flush) because you will see the joint. Instead, choose a technique like the one shown in the video (or others) to hide the joint and make the transition look intentional.