I’d skip DE tbh. I’d pull all bedding and everything from the coop, buy durvet 10% permethrin, mix in chemical sprayer and spray the entire thing down. I’d follow up with treating individual chickens with elector psp (spray under wings, behind neck, on vent) maybe twice in a 10-12 day span.I culled my roosters last fall, currently none and don't really want them. Hens seemed more stressed out with them than without.
Before I switch over to the new pullets I'm going to super clean the coop , maybe just replace roosting boards. I coat them in DE though so again seems unlikely they would be the issue.
Then I’d do permethrin spray as a preventative. Maybe once every 3-4 months. Nothing crazy like spraying everything. Probably just perimeter.
edit - this is if you can verify mites or bugs of some sort are an issue. This is a lot of work if it’s not. I wasn’t 100% sure I had mites but my chickens bounced back nicely after - less missing feathers, wounds from scratching, etc