Why do you talk about a smaller motor axel dia. you specified a certain motor with 8mm axel, my design is based at that spec and the motor is in the assembly.
If you want to use the "connector" you show in a post #69 you ned to design another end shield for mounting the motor.
Please remark the motor axel is axel for first stage planetary carrier. Look at the detailed view in post .#71 It rotates about a ball bearing.
If the motor is mounted with the axel end to end of SUN1 with your connector, its another Gear design and you most do it your self.
In #65 I delivered a finish Gear design and I see my effort to help finished.