Hi Sjef - When the z axis is at bottom is the critical point for stiffness. You need to have the z axis thicker at this point. A-A in image. You have compromised the z axis by considering the Makita lead, move the lead or use a 48V 600W DC brushless spindle. The 48V type is smaller and better in my opinion.
https://www.ebay.com.au/itm/200W-300...-/132273974879
You will be disappointed with using small tubes for motor standoffs. Better to make a ply base. They will deflect under the required belt tension. Peter
I've attached an image of one of my benchtop machines. Will cut plastic, timber etc no problem. Uses N23 motors. You can see the belt has a single loop. It uses T5-10mm wide belt. It was intended as a plotter, knife machine not a router. Look at T5 belt then you can use a smaller pulley. I also use 400 step/rev motors. More accurate if that's important. On bigger machines I use 16mm wide or 25mm wide belt.