Two things I did not see mentioned that I have had a lot of success with are the supplement Source and silk protein.
The silk protein is a hair care product that I buy at like a Sally's Beauty Supply. I mix about 2 tbs of the protein with about 2 cups of water in a spray bottle and mist it into mane and tail as well as over the coat. It is the most amazing detangler for the money you will ever find. Over the body, after a thorough grooming, mist the water mix over the whole coat VERY lightly and then go over with a soft finishing brush.
Source is a seaweed supplement that very quickly adds a lot of healthy nutrients to their diet. I have seen it bring out dapples in the coat of a chestnut mare in her mid twenties when that was the only change in her diet in almost 8 years.