Husker...
My dog growing up, Fred, was diagnosed with lung cancer at 14 year old. He was a lab/mutt. We decided to try the meds but they made him so thirsty and he would throw up all the time His quality of life was awful. So my parents took him off the meds figuring, he could at least enjoy what time he had left. Some days he was good, some he was so bad we had to hold him up to pee. He passed away one night, almost a year after the vet had recommended we put him down. We got another year with him and although some days were pretty awful, some days he was just like the dog he was before the cancer.
My advice is to use the time you have and make a decision based on the quality of life Banjo has right now. You know your pet by now and even if he can't speak, he can tell you volumes about how he's feeling. If he is suffering, then the best thing you can do is let him go peacefully. We chose to let Fred hang on and pass in his own time but like I said, it's something only you can decide.
Good luck and take care. I know how hard this is. It's been 11 years since Fred died and I still get misty eyed when I think about him. And I also still carry his dog tag on my key ring.