As I stated, the most common way to die diving is from a heart attack or other health incident according to DAN. Running out of air is a very uncommon cause of death in rec diving absent a primary factor like entanglement. So no, you are absolutely not reducing your chance to 0 by doing everything right. You are eliminating the chance of suffering a death from one of the things that doesn’t kill a lot of divers. An overweight old person doing an activity that stresses your lungs and circulatory system in an unforgiving environment is inherently high risk no matter how thorough their skills and preparation.
Double check your health insurance, many exclude rec diving as well.
Every single dive instructor has a story of seeing an old guy have a heart attack, myself included (he survived, barely). The only other death I know of besides old guy heart attacks where I worked was a young guy that had a heart attack.