N is very peaceful from popular methods that can be found
Inert Gas method is very peaceful when done properly your out within seconds
Shotgun to the brain probably is very peaceful as well but in a violent and ugly way
I have read a lot of survivor stories, people talking about it after consuming and before dying, and videos. I don't think it qualifies as peaceful tbh.
I believe it is uncomfortable (to be expected your dying) and can even be miserable.
I have read witness experiences saying they seemed to be gasping for air strenuously and convulsing. And also all the typical experiences like nausea, severe nausea, burning throat, bad headache, very fast heartbeat, getting cold, dizzy, loss of vision, lethargic and unable to stand, and of course vomiting. I have also read first hand experiences of it not being as bad as some folk make it out to be.
For me, the only thing that stands out as possibly really uncomfortable would be struggling hard to breath in combination with severe nausea/vomiting. But if done properly and you don't start rapidly vomiting back to back you will be unconscious eventually and sooner rather than later at that. That is the last ~40 minutes you will have to endure in this life probably less if we consider when consciousness is lost.
I don't think knowing this will stop me. I want to die. I really want to die. I'm not naïve and understand what this is all about. I have tried for years to sustain myself and get out of the wheel of suffering I exist in. I'm not doing this impulsively or before I have even tried to live a life in society.