Quote: I think crowding is at least part of the problem...even with a large run and occasional ranging.
What is your climate? Adding your location to your profile can help folks give better advice/suggestions.
I wouldn't treat for mites unless you know you have them...not sure what 'DM' is.
What are you feeding? If they are eating feathers they may well be low on protein.
My Feeding Notes: I like to feed a 'flock raiser' 20% protein crumble to all ages and genders, as non-layers(chicks, males and molting birds) do not need the extra calcium that is in layer feed and chicks and molters can use the extra protein. Makes life much simpler to store and distribute one type of chow that everyone can eat.
The higher protein crumble also offsets the 8% protein scratch grains and other kitchen/garden scraps I like to offer. I adjust the amounts of other feeds to get the protein levels desired with varying situations.
Calcium should be available at all times for the layers, I use oyster shell mixed with rinsed, dried, crushed chicken egg shells in a separate container.
Animal protein (mealworms, a little cheese - beware the salt content, meat scraps) is provided during molting and if I see any feather eating.