I'm curious about the main ingredient in Durasole - formaldehyde. The website says it "fixes the exfoliating sole" (keeps it from exfoliating?). Exfoliating sole is "dead sole" and the new sole growing from the live inner structures is called "live sole."
If you "fix" (glue?) the shedding sole to the live sole, it may be a temporary fix to hold the foot together, but may be sending the inner, live, growing structures the wrong message, like "don't grow." So, Durasole probably works, but does it allow the sole its necessary expansion?
In a dry stall, the horse stands in occasional/perpetual urine (or ammonia soaked bedding) which may eat his hooves as much or more than a wet pasture.
Apple cider vinegar (cheap, natural) dries out thrush. Might that help?
As you listen to your horse's cues, please remember that growing a strong new hoof out of weak, damaged hooves takes almost a year. Is he saying: "too much too soon?" If you ride on hard surfaces, is there a place where he can live on hard rocky ground?
May we see photos?