One diet I have not tried was vegetarian?
Dogs are NOT obligate carnivores and can live long, happy lives on a vegetarian diet. A co-worker, years ago before we had such varied dog foods commercially available, had the same problems as you and raised her hound on a vegetarian diet. As with human vegetarian diets it was not easy getting the right mix of foods and took a lot of cooking just for the dog but she did it. If you are sure that's the next step to try just do an internet search and you will find quite a bit on vegetarian diets for dogs.