The GaN hemt would make sense for a second board iteration where you drive the 75V straight through the diode with a very short (variable) pulse like in this EPC reference design:

https://epc-co.com/epc/products/evaluation-boards/epc9172

That way the 1.4 Ohm resistor doesn't eat like 60-80% of the energy (40amps drop around 60V for an instant 240W in the resistor vs the foward voltage of the diode which is like 1.5V or so for the 60W peak).