Brackets for reinforced shelves,
it is possible to mount them without reinforcements for small loads (already very resistant) but by mounting the reinforcement you have a greater load force. by mounting the reinforcement bracket, it is still possible to easily screw the screws to the panel because I left space for the screwdriver.
the reinforcements could enter with force helped by a light hammer

I used ABS GF by bambulab, so the yield is even greater.
you can use petg, or you can exaggerate with materials such as nylon

I used 6mm dowels

The bracket measures 200 x 140 x 25 with 5 mm holes and a countersink for the screws

you can easily mount a 225mm wide shelf