So think harder! Admittedly I did recently make something that turned out to be impossible to assemble...

Have you checked that the Z-axis belt clears the posts upon which the motor is mounted? If you decide to change the ratio it might not. Also, I wouldn't reduce the thickness of that motor mount beneath the motor, partly because it needs to be strong but also because it adds an extra machining operation which will increase the cost of the part unnecessarily.