i wouldn't add anything to drinking water in warm weather as it will discourage, or even stop, them from drinking. Red Stop sounds very dodgy stuff. May affect the eggs and hatch ability.
We use Smite to flush the eggs and mite out of the wood joints as it has a detergent content. Not a good killer though -we use Nettex Total Mite kill for that afterwards. Creosote we use on all the coops but mite will still nest in the bedding. You need a spare coop if applying creosote and I would recommend at least a month drying time and that the perches and nest boxes are not treated.
There is an alternative 'nuclear' treatment. A biocide called Elector, which we have found 100% effective in one application for a year. Then the mites evolve immunity and normal chemicals must be used for the following year. £70 is a lot for the small bottle but it has a long shelf life and we spent far more than that on other treatments. We have 8 coops though. It is sprayed liberally everywhere -no need to take bedding out as the mites are killed on contact. It affects their nervous system so they can't find their hosts and starve.
We have never used diatomaceous earth but is sounds like a good idea in a coop already creosoted.
In France the recommendation is to add potash (wood burner ash) to the soil baths. We add it to dry sand at 5 -10%. It is very fine dust and suffocates the mite on the birds, sticking in the feathers. Trouble is add too much and you may be facing respiratory infections S&P.
Plastic coops are probably the way to go, but cracks and gaps in those will still house red mite.