For some reason I don't like the look of the box section as brackets for X ball nuts.
You seem to be loosing a fair amount of travel in Y direction by having short rails and ball screw.
Think about how you will access grease nipple on Z ball nut once it's all assembled.
Will you put adjustable feet on the frame legs for uneven floors ?
Can't see how you will adjust belt tension for the stepper motors shown.