When Gracie got pano, I inquired very directly of the vet, and he absolutely 100% assured us that it is nothing we were doing or not doing, or feeding or not feeding. (Feeding a high grade food with moderate parameters, not grain free.)
I don't think the supplement you are giving is likely to do any damage, but the same vet, re Gracie, as Diva has some arthritis, tells us to wait until she is about five to start glucosamine and the like, as her body will produce it just fine while she is younger.
I would not worry over the amount of C in 1/4 scoop of the supplement you are giving (75 mg), relative to the size of a Doberman.
We were easily able to see the pano in Gracie's x-rays. You can give painkillers for it, though I chose not to do so on an everyday basis, as she did not have pain on an everyday basis. When she would indicate pain, I'd give them to her for about three days, then leave off until and unless she indicated more pain.
It should be good to be Dog.