@dandangerous, this is a brilliant writeup, thank you for sharing!
The spindle+VFD users among us went through a subset of these steps to wire the PWM to their VFD, but having this tutorial available so that router+SuperPID users can also benefit from automatic start/stop is great.
I think the config.h that Rob posted a while ago matches the GRBL version in use in the Shapeoko firmware, which unless I am mistaken is version 1.1f. So when I recently rebuild/reflashed my firmware to add hardware feedhold support, I downloaded that 1.1f version and applied Rob’s patch as is.