I have modified the GRBL 1.1F version of the 6 axis MEGA2560 and made my board
i have added 4 additional relay controls with g-code commands, in addition to cool/mist. (total 6 relay controls)
only have X, Y, Z, and A usable through settings.
i use one my homemade CNC with bCNC controller and works awesome.
it also designed for the higher amperage stepper drivers but i have also designed add-on boards for a4988/8825 drivers.
if anyone is interested in it, please let me know. i will send info//pics of my boards