Yeah, tried all that.
The only thing that helped was having the anal glands expressed at least once a week by a vet tech.
You can ask your vet to have the vet tech show you how to do it yourself.
My dog with environmental allergies had anal gland issues, once she started treatment for atopic dermatitis by a veterinary dermatologist all anal gland issues went away.
She does well on a variety of foods. The dog food did not appear to have anything to do with it.
PS: GSDs have specific anal gland problems related to the breed.
