I have had problems with the thinner boards (1.2mm PCB’s)not making ground, probably why he is going back to the 1.6mm boards, the retainer ring would bottom out and the driver felt tight but wouldn’t be tight enough to make ground.
Assemble everything outside the pill and see if it works….touch your power supply leads to the spring and ground ring on the driver momentarily to see if it works…
Maybe the FET is fried from too much heat?