Hi there,
great looking project.
I service and repair welding equipment including these wee plasmas. I think you might encounter some excess wear problems
by earthing the nozzle of your torch.
More sophisticated plasmas earth the nozzle so the HF can start, usually at reduced current, something like 20A to
prevent excess wear/overheating of the nozzle, this phase is called pilot arc.
When the arc is offered up to the workpiece the arc 'transfers' to the metal to be cut, the plasma uses a current sensor in the
earth lead and detects 'transfer' and then releases the earth off the nozzle and ramps current up to full current setting.
If the earth is not released off the nozzle a substantial fraction of the cutting current flows thru the nozzle causing wear and
overheating aside from depriving the main arc from its full cutting potential.
A hall effect current sensor and relay or IGBT would solve the prob.
Of course it may work well enuf as is without excess nozzle wear....