Looking really good, well done.

Be aware that if you turn a stepper using a drill it will generate power (like an alternator or generator). If it is connected to the stepper driver board this can be a bad idea!

For motor mounting plates you need to have slots not holes so you can tension the belt. 20mm or so slot is a good start but you'll need to decide based on the standard belt lengths available and making sure you can tension it. There are some online belt length calculators available so make sure you use those and place the centre of the slot in the nominal position.

The motor plates are usually pocketed with the motor set into the plate allowing the drive belt to run free over the top surface. You've got the motor sitting flush with the rear surface so you are having to cut all the material away in the middle to allow the belt to run. Not saying your way won't work but its usually done the other way to keep some strength in the bracket.