Hey, vet tech here. Unless an animal dies of something like an arrhythmia in their sleep, "dying peacefully at home" just... doesn't happen. Most deaths are, actually, pretty horrific and prolonged.
As animals age, organs wear out. Kidneys can start to fail, leading to increases in chemicals that are caustic and result in stomach pain in particular. This is why cats with renal failure or insufficiency will vomit and not eat. The kidneys also control red blood cell production, so when they start to shut down anemia is a possible effect. The liver can stop working quite as well, leading to build ups of various toxins that it is designed to filter from the blood. Cancers and arthritis happen as well. Cataracts are also common, and that WILL require the snake being switched to f/t.
I had to euthanise my blind yellow rat when he was 16. I was finding him upside down as he was highly disoriented, and he was no longer able to find the water bowl on his own. I had him skeletonized and apparently as he aged his bones either became brittle or he had the start of bone cancer because he has dozens and dozens of spots where the ribs broke and healed, or were healing, or hadn't started healing yet. So it turns out that he was in constant pain towards the end that I was not aware of, and that makes me feel really bad.
Our snakes, despite being predators, are also eaten by larger things. They are not going to show us signs they are in pain, because if they did that they'd be eaten in the wild. It's up to us to keep an eye on very subtle behaviour changes to try to catch things early. My boyo was always a lump so not seeing him move for 3 days wasn't atypical. But I found him upside twice and after that second time I decided it was time.