This is universally my experience as well. The more wild I allow my chickens to be, the less illness they experience. I think maximum exposure is the best method for having a healthy flock
I would recommend raising tougher chicken breeds and making sure that they have plenty of places to hide from predators. I have a large fox den on my land and they attempt to eat my chickens maybe twice a month, but I haven't seen them catch anything in years. Foxes aren't fast enough to catch...