I'd start by balancing out your horse's diet. Millrun (same thing as wheat middlings IIRC) is fine as an ingredient in horse food, but doesn't have enough nutrition to serve as a standalone feed product. Many of the skin problems horses pick up (greasy heel, rain rot, etc) can be completely avoided with a proper, balanced diet.
I'd restructure the diet to be high in forage (hay or fresh grass), fed at ~2% of your horse's ideal body weight at a minimum, or better yet free choice. Supplement that with a ration balancer for nutrition. (If you let me know what brands are available in your area, I can make some specific product recommendations)
You can also add in additional calories with rice bran, soaked beet pulp and/or alfalfa (cubes, pellets or hay). I know you mentioned he didn't like rice bran- I haven't seen any horses object to rice bran unless it was rancid, and rice bran goes rancid very quickly if it's not stabilized. It might be worth trying again with stabilized rice bran. A lot of horses don't like beet pulp at first, but quickly grow to love it. If you want to try beet pulp, you can try mixing in some molasses until she gets used to the taste. I haven't found a horse yet that doesn't love alfalfa