Mount over-the-range microwave without top cabinet
There are brackets made specifically for mounting a microwave oven on the wall. When you use these there is no need for a separate shelf. Do make sure to mount the brackets into studs and not just to the wall with hollow wall anchors.



(Picture Source: https://www.amazon.com/dp/B07FPFNFB1)

The set of brackets shown above are rated for 110 pounds when securely screwed into two studs.