Honestly it looks like a pretty solid design. I would stick with direct drive unless you feel a very strong need not to. Belts are fine, but add a fair bit complexity to mount and have low backlash. I prefer the PowerFlex GT2/GT3 profile (curvilinear) if I need to use a belt.
I would look at the gussets that link your rails to the vertical plates. I am missing how screws will attach to the carriages.
Your carriages are too close together on the Z axis. I would sacrifice a little travel for the extra rigidity. Going along with that, rigidity is a major issue on "router-style" machines. Keep the total height of the Z mechanism (vertical plates, Z axis, spindle, etc.) to a bare minimum. You will probably want it to cut 1/2" sheet well, rather than being weak for the rare job that needs a 4" thick piece of stock.