Hi, I’m so sorry to hear about the mites. The good thing is that you caught it. You are correct in thinking all birds will need to be treated. So will your coop and bedding. The bedding and nesting material will need to be discarded or burned,
do not reuse or compost it. The nooks and crannies of the coop will need to be sprayed with a permethrin spray (best option) or sprinkled with a permethrin poultry dust.
Do you have a vet to work with? There is a prescription mite treatment that seems like it may be the best option for your scenario. It goes in the water and will work as the mites bite the birds. It will work for both northern fowl mites and red mites. Here is a link to a thread with some more information:
https://www.backyardchickens.com/threads/using-exzolt-for-northern-fowl-mite.1681455/#post-28953407
Otherwise, since it is so cold, a poultry dust (permethrin powder) may be your best option. I do not have a large flock, though, so let me tag some others who may have better advice on treating such a large flock.
@Wyorp Rock
@Eggcessive what are some good ways to treat a large flock for mites?