So I had part of the answer all along on one of my tabs:
“I think the easiest thing would be to connect a relay to your laser and control the relay from the spindle pin. Then you could use the normal G codes M3 (enable spindle) and M5 (turn spindle off) to control it.”
Looking at the pinouts, the “Spindle Enable” is pin 12, which helps a lot, but which pin on the GRBL shield would it be?
