First of all, cyanide is not painless. It's extremely painful, but in high doses, it can take as little as 2-5 minutes to kill. As for painless poisons, I don't think there's any that's readily avaliable right now. With nembutal being on short supply and other options being extremely hard to obtain. So there's just the usual SN and hanging stuff, there's also yew poisoning that you can try, but I can't find a reliable online source for it and the success rate is not high.