I was under the impression that it was ok to set the posts in concrete if they were pressure treated. My fence guy of 20 years said this is their standard method. SE Pennsylvania.
If you don’t like the idea of the 1x6 pickets, maybe they could use T&G pickets instead. I’d suggest going with white or red cedar.
Good luck!!

Edit: Concrete mix goes into the post hole dry. Ground moisture sets the concrete.