Pfffttttt....

I took mega 2560 board for lathe, since I need extra timer for chuck rotating, when turning basic shapes...
So, now with potentiometer & switch & adc input + timer5 I can rotate lathe chuck without needed G code info for Y from gbrlgru.

Oh, and there is a MAJOR error on your thread code. You can't EVER stop spindle, when blade still cutting. Have to
rotate an one extra time to make "fake thread" OR run X away from stock before turning chuck off. I broke 2 blades already
because of that.

And now I have modified GRBL code, what works with your grblgru. My grbl takes your gcode command, divides y-axel value by
360 and when it reporting back the position for grblgru, it multiplies y-value again by 360, so I had to lie for your software...
That way I got your 360mm/rev mistake fixed and I can use reasonable stepper / rev values with grbl board & steppers.