I'm so sorry to hear that. My dog died from cancer a few years ago, i know how hard it is to see that. But before we take a pet, we know that he'll probably die before we do. That's the good case. the most important is knowing that we gave them the best life possible.
Your dog had and is still having a great life, surrounded with love. I agree with you not wanting him to suffer from the treatments. That's not how he deserves to go.