Towbo, you got some good recommendations, some totally off base recommendations and now the rvnet thermodynamic and heat transfer experts are showing up.
Barring any of the obscure suggestions (although good to check if not too obscure), sounds like your set up is perfect...just about.
Starting out, you're at the upper end of both the tow rig and trailer brakes coming down long mountain passes. Aint to diesel dually which you're keenly aware of.
You're obviously not totally smoking the trailer brakes.
3 things to do, in order.
Back off the trailer brakes just a little. (But I'd rather have the trailer do as much work as it can on it's own)
Use your vehicle's service brakes a bit heavier.
Or go a bit slower.
You absolutely want to drop gears like your doing and use as much compression braking as you can and when it's time to brake, get on the brakes quickly, drop speed to below what you need to and then stay off them as long as you can.
The ridiculousness discussed about it being the same if you lightly ride the brakes all the way down vs hard braking with a rest period is just that.
Otherwise, I'd still prioritize as much braking as possible with the trailer, even at the expense of the trailer brakes.
Burn up trailer brakes, they're cheap to fix and you can still get home with good car brakes. If the opposite happens, it costs more and you can't get home without fixing them.