Have you tried feeding Apple cider vinegar? I’ve found that it works wonders for horses that are itchy! My gelding is terribly allergic to any type of bug bite and will rib himself raw but about an hour after eating the Apple cider vinegar he feels much better. My mare who has a thinning mane ,because she scratches, also seems to do better on it. she doesn’t stop scratching altoether but her hair does grow in faster and thicker. It’s fairly inexpensive to try. I buy the gallon jugs of organic Apple cider vinegar from Costco and depending on the horse I feed up to a cup a day. I hope this helps, I know how difficult it is to make thin makes look nice for shows.