I'm so very sorry about Tresoro. What a beautiful pup he is.
I can understand you state of mind so well. When my first Golden developed breast cancer, she had surgery and the tumor was back in six weeks. She went through chemo, which didn't solve the problem. Finally, when the vet told me that it was spreading to her lungs, I made the very hard decision to let her go. To this day, and this was over 20 years ago, I still feel that it was the best I could do for her, as hard as it was for me.
Talk it over with your vet. Whatever you decide to do, we know it will be based on your love for your precious pup.
Hugs to Tresoro and you.