Not sure what width you've been using, but have you considered that if you increase the width, you need less pressure? If you double the surface area of the tire 35 psi will you the same amount of support for your heavy load as 70 psi would in the smaller tire. Not to mention you'd get a lot slower wear just because there is more rubber to go through... I feel like finding the widest possible tires should be step number one, then looking at hardness and thickness.

Maybe you already know all this, I just noticed that you ruled out a couple tires due to pressure. Not sure if width/overall surface area were accounted for or not. Thought I'd chime in for the hell of it.