Lethal injection is, I believe, more painful than hanging. As are most of the other methods we transitioned to away from hanging.
Firing squad, guillotine and such are acceptably painless, done right, but are too messy for people to accept it today.
I propose nitrogen asphyxiation as a fix for the over-complicated mess that is lethal injection. If it was decided that firing squad, heck, captive bolt pistol, was enough, I'd go for that.
It meets the 'no unnecessary suffering' standard. It's simple. It doesn't require specialized medical training and experience. It doesn't depend upon chemicals that are easily cut off.
You can go on about old methods of execution all you like, but they've already been ruled out.