You need to treat both housing and the birds.  Dust bathing alone won't treat the bugs that are on the birds if there's an infestation.
You can either use your spray on them according to the directions on the packaging or use a Permethrin poultry dust.  I find it at TSC.
Personally, I like dust for the birds.  I put it in a sock, which makes it easier to apply.  Grab a bird, tap the sock onto their feathers to release a fine dust, then work the dust into the feathers down to skin. I dust all over, but try to focus around the tail, under the vent, between the legs, under the wings, etc.  Places bugs like to accumulate and hide.
Repeat in 7 day intervals.
During treatment periods, I would use the minimum amount of bedding since your having to clean all that out.  Spray down all your nesting boxes, nooks/crannies of your coop, the roosting bars, etc.   Mites hide in the cracks and come out to feed at night, but a bad infestation, some will stay on the birds.
I'm sure it's frustrating, but keep at it.  I think you may see some improvement if you dust the birds too.