Our 2017 M325 has the 900W High Pointe microwave and we and to replace it with a Convection / Microwave. the cabinet opening dimensions are 20" wide x 13.5" tall x 15' deep. Is there a unit that will fit that space?
I have seen 24 inch microwave/convection ovens. Here's one.
If you modified your cabinet to remove the door and that stile a 24 inch would probably fit.
You will/should need to vent a convection to the outside in order to get rid of the heat they produce when cooking.