You definitely want to completely clean out the coop, removing all the feathers and shavings/bedding. We put Distomaceous Earth in the new bedding on the nest boxes and in their dust path. This will kill the lice and mites. However, we also give them an oral dose of Ivermectin. It’s tricky to get sometimes, so not always the best. We have the one for large animals and just give them a small dose. Great thing is it can also be used for worms, scaly leg mites, and other parasites. So it had multi usage. Just remember to treat the birds again two weeks later to kill any lice or mites that hatched. Since you can’t kill the eggs usually.
I don’t know if this will help. It’s just what we do when we get mites or lice. I hope your flock does okay