Excellent work Zig.

Rigid couplings are not normally used, for various reasons. On my machine, try as I may, it was not possible to machine the motor mountings
precisely square and parallel so proprietary flexible couplings allowed my steppers to run without stress.
One other point you may wish to consider is incorporating a thrust bearing between your motors and screws. The motors have dimensional tolerance during manufacture and this is usually taken care of (by the manufacturer) with the use of one or more crinkle type washers fitted behind the rear bearing. This will allow the shaft to move axially under load and may cause you problems with maintaining an accurate tool-path later on down the line.
These comments are in no way intended as a criticism, you are doing fine work with building your machine and I am sure your video’s will be of great help to others. Keep up the good work my friend.
Tweakie.