Maybe JB can walk you through a more thorough appropriate worming protocol.
I’m in hot humid Fl. and generally if the diet is adequate, skin conditions aren’t a problem.
The ONLY time I’ve had rain rot or other skin problems is when I boarded and my horse was on Nutrena feed. We didn’t have a choice and the barn had many, many problems w/ skin problems on most of the horses. I’ve never used it since. Look into Triple Crown feeds.
Also I’d suggest you bump up the quality of your hay. Maybe start w/ half Alfalfa instead of Bermuda. Alfalfa can really improve their overall condition. And weigh your hay. 1.5% to 2%
of body weight should be their daily minimum. If they don’t have pasture then you can supplement w/ the lower quality hay just for munchies.
I’d say the skin problems are a result of low quality and quantity of your feeding program.
I’m glad you got another vet to help out.