Have environmental allergies been ruled out? What you describe sounds like that’s what your dog may have vs food sensitivities.
They usually start around 1-2 years old and get worse with age. Environmental allergies tend to wax and wane making it almost impossible to tell which foods work or not.
Everyone blames the food, but I have found my dog tolerates a lot of different foods since beginning allergen specific immunotherapy (desensitization). The treatment tends to be lifelong, but no more ear infections, rashes, anal gland issues (5 years in)
The initial testing, intra dermal skin testing is expensive, maintenance isn’t that bad.
For best results, an accurate diagnosis and treatment options that work, I would make an appointment with a veterinary dermatologist. Allergies are complicated and often require the expertise of a specialist.
