Lower voltage will give you less speed and torque. At some level it will be too low to even spin the motor.
I'm waiting for my QS138 as well :) Did you get the QS138 v3 with internal gearbox?
First of all great work! How important is it that the motor is completely aligned? Does a small angle make a big difference?
My build is starting in a month and I was hoping to weld the mount since I dont have access to a CNC. Guess alignments will be a bit tricky.