I know what you mean. I think it's something we all fear and worse- we don't really know what to expect. Some people seem to pass fairly easily from SN while others don't. The overdose of meto kind of frightens me too- especially since mine is out of date now.
Still, I try and reassure myself by saying my 'natural' death probably wouldn't be all that pleasant either. I don't actually know any family member who didn't suffer before they departed. Granted, a lot of them were under the effects of morphine and even heroin in a hospital when they finally passed but their journeys weren't smooth.
Also, I've had chronic pain in the past and waited it out in the hopes I would die. I thought it was a heart attack but it was gallstone related. The symptoms were actually fairly similar to SN though- perhaps worse by the sounds of it. I felt a terrific cramp around my diaphragm area (that was the worst of it and to be honest- SN doesn't sound as bad as that). Also felt a shortness of breath which made me panic. Still, I didn't call for an ambulance- just cursed God that if he was going to kill me to get on with it (if there is one) but sadly, it passed but kept repeating until I couldn't stand it anymore and sorted it out.
I guess, at least with SN, we will be kind of expecting some of these things. Honestly- it was terrifying to be hit with a pain you feel like you can't cope with and have no idea what it is. Again though- it's odd because I have spoken to other people who had gallstones who suffered the same and others who didn't. Maybe it's to do with our pain tolerances or the particulars of what the damn gallstones are doing.
I do feel so resentful though that we all have to go through all this. I do agree that assisted euthanasia sounds so much better for everyone concerned. Still, sadly- this is what we're stuck with. It will probably be a 'nicer' departure (and hopefully more reliable) than some of the methods I have considered over the years (like slashing wrists and OD-ing on paracetamol.)