Nobody knows, because those smiley face pain scale reports are real hard to collect from the other side.
I can sympathize with your quest, though… for a decade I was writing that same story in my head. That’s what I told myself, anyway.
Seriously, though, I doubt any method of death is truly painless. We feel pain in our central nervous system (brain), and nobody can tell how long those nerves keep shooting out frantic pain messages in the event of an emergency system shutdown. Just because someone can’t tell you they’re in pain doesn’t mean they don’t feel it.
This may be of interest:
The search for a more humane execution is flawed because there’s no such thing
As an aside, mystery authors are so hung up on finding a clever twist way to commit murder without anyone finding out whodunnit. The perfect crime. But what I wanted so badly was a way to commit suicide without being found out.
Your average Joe will fake an accident, but my fantasy was to fake a murder. That is, to have a character die by their own actions, and have someone they hated take the fall for it.
Inception had a pretty cool take on this.