Nice Design. It is definitely giving me some ideas. You know it reminds me of how I collimate my Dob Telescope. :)
Question though. That screw on the plate. It looks like it goes right into the motor. Have you thought about how much clearance you have to the motor? Like how long the length of the screw is. The motor on mine sits about 10.4mm, give or take a smidgen, from the top of the corner bracket. That may be something you will want to double check if you haven't already.
Coming back to the reminder of collimating my telescope. It has three knobs for adjusting the mirror and three for locking it in place once everything is collimated / aligned. I wonder if it would be advantageous to put three locking points on a design like this. That way it would not move from say any vibrations. That may be something to test out.
Anyways... I really like your design. I'll have to come back and see how it works out for you. :)