The end is usually quick and painless. We figured out how to cheaply do that... I think most of the problem is how they live their lives until that, and it depends on the farm. I've personally been to pig farms, where they seem pretty comfy. But I'm not under the illusion all farms are the same.
Quick & painless death is a marketing myth. And as you admit, at best, it's most of the time. Even once going wrong is not acceptable. Ask yourself why the slaughterhouse industry refuse to publish stats on animals that suffered at the end?
Fear & trauma of the animal starts when taken from the farm. Betrayal by the "loving" farmer, roughly handled, no water in transit, no care whatsoever, smell of death as they approach, kill line explicitly blocking choice between either fight or flight, electrical prodding in their privates.
Everything you are describing can be fixed. Starting with the killing itself - that's a skill issue. Better employees, with better training. There are also ways to make the animal drowsy, or unconscious. Veterinarians do it without "spoiling" the meat. It all comes down to money and public opinion.