I'm sorry for your losses.When I noticed the mites, I did a thorough deep clean of my hen house. It was an all day event! And then I used DermaEarth food grade in the hen house and rubbed it in each of the chickens wings. I haven't noticed any mites
Getting a necropsy would be a good idea.
In the meantime, re-check the birds and housing for mites.
I know a lot of folks use different products, but if there's an infestation, DE is usually not as effective a treatment for external parasites as a Permethrin based product.
What did you deworm with? Is there a way to take a sampling of poop to a vet and have a fecal float just to rule out worms and Coccidiosis so you know these are unlikely contributors to decline?