I understand your hesitations completely! I think you are right to think carefully about a significant surgery on an older bully - who is significantly overweight. I am not saying that to hurt you but 20 pounds overweight on a person is quite a bit, so on a dog, it's taking a tremendous toll on the body and all its functions.
I have an older bully too - he is 7.5 and, in his prime, he was 83 pounds but is now down to 71, which is where the doctor wanted him. Two pieces of advice:
First, as an owner of an older bully, I most likely would not have any kind of surgery on him at this point. Not only is he not as resilient as he was when he was young (I'd be afraid it would just exhaust him) but I'm not sure he'd do well during surgery and recovery. My bully is less active and less tolerant of so many things that he once was but you need to evaluation your bully's overall health and activity level to make a good decision.
How is your bully's overall health and demeanor? Do you think that your bully would weather surgery well?
Second, as someone who has had not one but TWO bully knees blow out on my first bulldog, Bentley, I can tell you that the first thing a good surgeon would recommend to you is to get your bully to an ideal weight, see how he does, and then do surgery, if indicated. Bentley was 10 months old when the first knee went and 21 months when the second blew out (they usually both go - one after another) and he wasn't in bad shape but the first thing the doctor told us to do was to get him down to 55 pounds and never let him above it to give him the best chance of a full and normal life.
We had both knees repaired and he did very, very well and never had another orthopedic issue until his death (which was only 6 years old) but it did take a toll on him. The recovery was long but he did regain all his mobility.
I am sorry you and JD are going through this. There are no easy answers but I think you're doing the right things by getting opinions and ideas about what your options are.