Xanax would probably help curb some of the SI, but not all. You gotta remember, SI's a completely natural and human thing to have. Everyone has it! It's not easy to overcome it even if you're as suicidal as they come.
As for your title, if you're asking for the most painless way to go, definitively, I think everyone would agree in saying opiod OD, followed by N, then SN. Opiod OD is the most feasible one you're looking at followed by SN. N can only really be gotten in South America unfortunately.
Opiod ODs have the benefit of making you feel blissful as you go out. Having OD'd before, I know the feeling that you get when your breathing starts to slow, everything in your vision gets a little blurrier, a little darker...it's a very, very peaceful way to go and if anything, it actually feels good. So long as you can keep it in you, anyways - opiods might make you throw up a bit, so if you do go for them, lie face down so you go naturally from your breathing stopping and from CNS depression, not from asphyxiation, or else that might actually still hurt a lot. Regardless, you'll be high as a kite. Main downsides are that the cost is a bitch and a half, so you gotta really be into buying that fucking ticket. Another one is that if you survive, it's not all sunshine and rainbows. Anyone who's survived will tell you about the itching - prepare to be covered in rashes and your skin made a raw red from the amount of scratching you'll be doing.
I don't know much about N but I've heard it's similarly as peaceful and painless, works within the hour. SN is less painless, but only mildly - burns on ingestion a little bit, like alcohol or stomach acid I suppose, and you're likely to feel your heart racing, vomit, and feel your breathing slow, but it's not a bad way to go out, plus it's cheap, hence why so many people go for it. Unfortunately purchases are being cracked down on so if you're anywhere in Europe it'll be tough to get a hold of.