Hi,
Have you thought of changing to homemade food?
Two years ago, my (then) 14 year old English Bull terrier cross bitch was looking very old, lethargic and showing various health problems which I put down to old age. I was trying to mentally prepare myself for that "final" trip to the vets.
While I was procrastinating, I changed her diet from her expensive, specially formulated for senior dogs, processed dog food, to a share of whatever meat and vegetables we were eating ourselves. The improvement in her health and energy over a period of six weeks was phenomenal!
That was two years ago. She is 16 now, and as energetic and healthy as she was at age six!
I now feed her mainly raw meat and organ meats (sometimes cooked). cooked vegies, eggs, yoghurt and cottage cheese, small amounts of cooked whole rice, small amounts of fresh fruit. I give her 1x fish oil capsule daily and I add olive oil or safflower oil (or sometimes butter) to her food if the meat is very lean. I would give her bones, but her teeth are worn down. (She used to eat a lot of raw bones when she was young).
Could be worth a try?
Best of luck