Bladder stone surgery

My cat is 15 and has kidney disease. I have him on a perscription diet, sub fluids, BP meds and azodyl. The initial bloodwork he had BUN level at 92 and CREA at 5.5 which is apparently quite high. After doing the treatments for a few months the levels have not changed. I was previously told if the levels didn’t lower it would only be a matter of time and not much more could be done. He now apparently has a slight heart murmur, a bladder stone and lost 1lb from 6lb to 5lb. My vet suggested he go to a cardiologist to check on his heart and get surgery for the bladder stone removal. I’m wondering if I should proceed with the surgery? I know he is sick and his situation is incurable and based on all the information it seems like it wouldn’t prolong his life much more.