I wanted to know this too. First of all, if it's really actually painful, or just uncomfortable. Because with full suspension your airway gets cut off, so you struggle for breath until the moment you're unconscious. And I really don't want to be my last moments to be panic (because of survival instinct of my body) but I have no other available options. Partial hanging COULD be an option, but sorry, I've seen way too many people fail that here. I can't risk failing. So full suspension is what's in my mind right now. And I feel like that's the way I'm gonna go.
So, about painkillers, I'm not sure if they would work as in for the pressure around your neck (that could possibly be painful) or for the struggle for air. I feel like there's nothing you can do about it being uncomfortable unfortunately... The pain doesn't scare me, but the thought of spending my last seconds in panic does. No matter how much one wants death, the body never wants it.
You could always get drunk and high off your mind, but as for myself I am unsure if I'd want to spend my last moments like that. I feel like sober I will remember the reasons I want to go more, so it will make me panic less.