Here's the first part done... the stepper to pulley coupler.
I milled a flat spot on the 10-tooth pulley, which mates with the coupler. It's a tight press fit, and also has a set screw for good measure. The stepper shaft also has a perpendicular pin going through it, so I have a tight slot in bottom of the coupler to mate with this pin.
There was no way I could mount this 10 tooth coupler on the 5mm stepper shaft... there wouldn't be enough metal left after machining the hole hahaa... I initially was going to have the pulley on it's own axle and bearings, then a coupler to the stepper - as you normally see. However I decided to make best use of the steppers bearings and to design a coupler to mate on the outside circumference of the pulley for a rigid solution. This was a far smaller footprint and less mechanically complex solution. Time will tell if the solution has a low enough deflection, or even if a minor angle offset with the stepper mount will suffice.