Saw your other thread:
https://www.backyardchickens.com/threads/spraying-with-permethrin.1305027/#post-21269223, and this one. We've had lice and mites, but neither problem was extreme. My kid handles the birds, even "cuddles" the most docile ones like babies, so we get updated on the condition of the birds fairly regularly.
We've sprayed 2x with permethrin. Cleaned out coop and sprayed roosts, walls, floor, nest boxes. Then, we catch each bird, one person holds bird upside down by legs (wearing safety glasses to shield eyes and gloves), and other person uses a garden sprayer (with the permethrin solution) to spray vent, under wings, and between legs - then place them in the coop with access door shut. Catch another - repeat. Since we catch the infestation early, we are targeting the areas prone to the infestation. So far, so good. However it will be a constant concern until we can keep the wild birds away.
Another measure we are undertaking is to add wood ash to the run and especially the common dusting areas. Luckily a neighbor has been cutting down some trees and burning them, so we have asked him for his wood ash. Overall, the birds should have some areas to dustbath in. Dry, loose soil/sand/ash mix. Our coop is elevated, so that area stays dry and the birds like to hang out there and we've added ash. However, they have some favorite areas in the exposed run to dust bathe, so we've targeted those spots with wood ash. The birds dust bathe to try to remove parasites naturally.
Good Luck.