Almost home after ~2000 miles - towing with a 1/2 a ton

I suggest that the tail will always wag the dog if the tail is heavier and has more sail area. Simple physics.

Getting a bigger truck will typically "call for" a bigger trailer and the discussion will start over again.

The solution is to know the rig and then drive at the speed that you are comfortable with. Upgrading the truck is often not practicable and is not the be all and end all to this issue.