What would be thes best option for drywall?

We built the shelves and were planning an outside corner piece to cover sheetrock, now the family is not exactly happy with that and wants more options.

Drywall guy said put 1X or casement around the perimeter and he could finish to that.

The issue is the center and horizontals. How to finish other than outside corner trim or two pieces of small tapered trim meeting each other? Some of the MDF pieces are shy of the framing, so there would be a gap with flat trim on the face