Hummm, I do a lot of hp too. To make the soaps harder or harder faster the biggest change I made was reducing the water content from 38% to 27%. It didn't make it more difficult to mold though because I use oven HP at 100C which makes it more fluid because it is quite hot. I also add salt to the lye water, which quickens hardening by adding extra sodium ions as soap is a salt of fat. It's not particularly humid where I am, I bet humidity would be a bugger to deal with storage wise.
And like you said hard waxes and butters would work well too.
But in my opinion the best way reduce softness is to use the min of water in the initial making I think 24% is the absolute min, or the lye will not dissolve properly. I found 27% to be the best.