If I were in your situation, I honestly don't think I could do it. Not because I wouldn't love the animal and want to keep them alive, but because I already saw my father pass away from cancer. I know I couldn't do it again with anyone again, human or animal, at least not for a long time. It's incredibly difficult to go through, and after hearing what my father had to say about it all, I realized he hated how much money we spent on him just to extend his life by the smallest bit. I understand the situation is different for animals, but I know it would be way too hard for me to try and get through. I'm happy you're doing all you can though. That dog is incredibly lucky to have you.
Sorry to hear about your father. When Hemingway (my dog) gets to a point when he is in pain, we will put him down. Till then it's about smothering him with affection. He hasn't been surrounded by this much attention since he was a puppy. Thanks for your kind words and welcome to Hubski.