My dog was medium sized--45-50 pounds, but hopefully relevant. She was 16 when she died. The only thing I would have done differently is had her put to sleep sooner. I feel like we waited two or three weeks too long and the pain/senility was too much. My DH disagrees, because she was healthy right up until she wasn't, but I still feel bad.
Now my other dog, she is 17, again 45-50 pounds, and she has mild arthritis. We do the pain meds. She also had a malignant melanoma removed last year, and we are doing the waiting game seeing of the cancer spread or returned.
We, too, chose not to go aggressive with treatment besides removal, and we did do the biopsy just so we knew what we were dealing with.
Our vet gave us good advice: when they show pain, they're in a lot of pain, and when they can no longer do doggy things, then it is time to consider options.
Just keep an eye on your dog and look for declining symptoms. Exercise as long as he will tolerate it, keep the weight in check (bad for arthritis), and