Another thing that you might consider is a vet nutritionist consult. I have used them for sick pups and been very pleased.
You can find one at:
American College of Veterinary Nutrition
Many do telephone or email consults. With one I had a couple of years ago, my vet did all of the initial contact and sent my pup's records. I then got a home cooked diet made especially for him and while he only lived about six more months, it was longer than would have been had I not done that. Well worth it! Actually the cost was not much at all...$150 for consult and the recipes.
I agree with you, that the most important thing is to keep the quality of life as long as possible.