As long as the relay clicks on and off when you want it to and doesn’t go on and off like crazy (if a pwm signal was sent to it) you should be fine. I would be hesitant grounding the machine and the arduino/ramps stuff together. I think the relay thing should be fine based on what I read in the “Spindle power control” thread in the hardware section of the forum.
On our Torchmate at school the pierce delay is what decides how long the torch stays at the starting position while getting the arc started. I have no clue what you would change in estlcam or any other software for that.