So sorry you're facing this diagnosis. One point I'd make from my experience: unlike with human cancer patients, chemotherapy in dogs is not traumatic and sickening to the dog...at least this was the case with one of my basenjis who had brain cancer. The veterinary oncologist explained chemo is a different process in dogs. Jasper felt much better with chemo - the benefits were almost immediate - and his life was extended by a year.
I totally agree - don't prolong life if it's not good-quality life. Jasper had other medical issues including Fanconi, and the time came when he lost the will and energy to live. We didn't let him suffer. He just wound down. Rather than try to get him to eat, we had a vet come to the house to euthanize him. It was a peaceful death and Jasper was not afraid. He just drifted off to sleep in our arms. The other dogs were with us and didn't worry and search for him as sometimes happens when a dog leaves for the vet and doesn't return.
Another basenji, Keiko, had massive stomach cancer discovered during diagnostic surgery. She had started vomiting daily and the vet couldn't find a reason, hence the biopsy. We opted not to wake her up from anesthesia because there was no hope for good quality of life. Very traumatic for us, but not for her. We miss both of them terribly but at least we did all we could to give them a good life. Both were rescues.