I have been in the same situation with 2 of my Boxers. The first thing we were told to stop was the monthly grooming. We went from every 4 weeks to every 90 days alternating a coal tar shampoo with an oatmeal colloidal shampoo(depending on skin condition). If your dog gets dirty there are wet wipes made for dogs that work great in between baths. The Benedryl in combination with the special shampoos really help a lot.
The second thing we learned was that like people dogs can develop food allergies later on in life. I had a 10 year old dog that developed an allergy to chicken after 6 years on a chicken and rice diet. Other ingredients to watch out for are any glutens and corn meal. I'm surprised your vet didn't recommend a special diet dog food as a precaution.
If all else fails find another vet. Believe it or not I found my current vet through a local Boxer Rescue. You can look online for a shih tzu rescue in your area for advice, they love to help. Above all, please remember that although these things worked for my dogs you should always check with your vet before changing your course of treatment. It can take months before you see any real improvement. I know how upsetting and frustrating it is to see your dog go through this but have patience. I've learned that a calm demeanor can be the most comforting thing to your little loved one. If you need any help email me firstname.lastname@example.org and I'll send you my number.