This would be good point of reference regarding the stepper motor choice, etc: sourceforge.net/projects/arduinoascomfocuserpro2diy/

Good question. I'm building mine as we speak. The electronics is the easy part. You need to refer to the attached document page 22 to know which motor suitable for your scope and start putting the plan together.

I'm still using the same motor I used with the adafruit motor hat. A smaller nema 14. It has plenty of torque at 12 volts. However, I have purchased a 12v nema 17 motor with 0.9°/step (400 step/rev) as an optional upgrade. I'll decide what to keep once I've used it a bit more.
As for attaching it you have options that you will have to work out what is best for your case.
You can:
Direct drive the focus shaft for zero backlash, but may not have the step resolution within the CFZ (critical focus zone)
Direct drive the fine focus knob. May introduce backlash, but increases step resolution.
Gear driven focus shaft or fine focus knob. Minimal added backlash, allows for gear reduction.
Belt driven. Use fiber reinforced belts with enough tension to avoid slack when changing directions. Belts also seem to slacken in the cold.

For hardware, there are shaft couplers and motor brackets. But truly the best asset is access to a 3d printer to produce custom gears, mounts and enclosure.
For connections I use DB9 sockets that can be screwed on.

If at all possible, can the Abort feature be added to this driver? Thank you kindly.

That sounds like a good idea. I will look into it!

I have a question. Why is this most excellent driver not part of the indilib on GitHub? Not even 3rd party?

I ask this for purely selfish reasons. Whenever I reconfigure my collection of Pi devices, I end up doing a git/compile for some of the things I need. This is especially true now that I am running Pi4 rather than Pi3 devices. While it is convenient enough to go to your site on github and do the same, it would save a step or three if it was associated in with the indi stuff. Also, perhaps that would make it unnecessary to create an "off-label" device during the Ekos setup. Sometimes that routine in Ekos can be a bit tedious.

Sorry for not getting back to you sooner. I've been away from astrophotography for the last few months. Nothing but clouds here in northern Michigan during the winter.

I wanted to fully develop and get testers for the driver before trying to get it merged into INDI. Now that it's mostly feature complete, and reasonably well tested, I guess now is the time for getting it merged into 3rd party INDI. I'll see if I can get to that sometime over the next week or two.

Clear skies!

Thank you. That will simplify updates nicely.
Have you had a chance to develop the abort feature?

No, I haven't. I should probably do that.

Let me just say that I have used indi_wmh_focuser for a very long time now with no issues whatsoever. Thank you for your work and for rescuing me from the land of Arduino.

+1. This is great stuff, Kevin. Thanks too for all your help getting me up and running.

How can I modify the code to use different step modes such as Single, Double, Microstep 8? And how it relates to the hardware mode switches vs mode selection via software.