5mm aluminum would make a good XZ plate. It’s a bit thin for 10mm M3 screws into the rails, but a washer would easily make up the difference.
The same for the YZ plates. You could then use shorter M5 screws. However, I don’t see any problem with the 15mm MDF. The XZ plates will benefit from a little extra stability, and the flatness offered by aluminum, particularly that part which relies pn both sides. The YZ plates have less need.
Oh, take note that you will need to move your Y belt guides when you change the XZ plates.