I really don't see where the thickness would have any bearing on this
problem since when released fully, even with the OEM disc, the warping drum
still is in full contact with the upper face of the friction disc. T

The drum isn't lifted off the disc, just less pressure from the engagement
nut. Zero clearance when free running. Sight pressure from the weight of the
drum, to increasing pressure as the engagement nut is tightened.

Thanks

Steve
s/v Good Intentions