I understand that even if you start off with a perfectly smooth road, wheels rolling over it will eventually cause the washboarding. Smooth gravel and sand is the worst. Crushed gravel that’s a mix of coarse and fine will interlock and resist this effect.
It even occurs on railroad tracks, called “roaring rails” and I have seen corrogated surfaces develop on roller bearing races.
If I had several miles of this road that I had to drive on daily, I would consider getting an on-board air compressor and deflate my tires to about 15 psi to drive that stretch of road and then use the onboard compressor to reinflate my tires back to normal when reaching the paved road. That way, the tires and not the suspension system absorbs the bumps.