There is no execution method in the world that will be given to a willing participant, almost by definition. The specific point and debate in this thread isn't about whether or not execution is right. Most people on this forum certainly are at least skeptical of capital punishment. I certainly am against it.
The debate instead is, "given that capital punishment will occur because Southern states are the way they are, which we can agree is horrible in and of itself, what is the least bad way to do it?" The discussion around which execution is least bad is valuable from the standpoint of harm reduction. Currently, choices are what exactly? A multistage cocktail of euthanasia drugs that paralyze the executee before stopping their heart? The electric chair? Firing squad? Hanging? Beheading? Everything you pointed out and more are applicable to these methods as well.
You might argue that this makes any execution method unethical, and you're right! Congratulations. You agree with pretty much everyone in this thread.