Try adding a teaspoon each of cinnamon and ginger and sprinkle with loose Himalayan rock salt. I mix the cinnamon and ginger into powdered supplements, where I don’t have the option of using pellets; then mix everything together with feed and timothy/alfalfa pellets, and sprinkle salt on top. You might also try replacing probiotics with Smart Gut or MVP Gastroplex pellets.
Good Luck. I had a 28 year old TB with same issues, and the spices encouraged him to eat.