Sorry for your and Chance's suffering. I know how hard this is and how you are racking your brain for a clear answer. Here is the way I look at it. Given the potential severity and his age, from MY perspective, you are in a range where anything you choose is okay morally and ethically and spiritually. There are several courses you can likely take and I think all or any are okay.
My Aussie recently was gaining lots of fluid in her body. The vet thought cancer or congestive heart failure. Diuretics and prednisone got us a good month and then she died relatively fast. My decision, given her age and the good life she led, was pallative care. I never did tests to see what the problem really was because it didn't factor into my decision. I suppose others might have tried more, and that is fine. But I knew that I would not consent to surgery, the rehab would have been too much for her.
My cross to bear.
I wish you well and again, I think you are in a range where anything is okay.
Dutch Shepherd - Ptygo (Tee-Go) de las Flores