So to preface, I am absolutely and without reservation against the death penalty, so any state-sanctioned murder is unacceptable to me.
That being said, if they’re going for painless, why not just a captive bolt stunner the their brain stem? Like, having them lie back in a massage table with a container for the blood (heaven forbid the audience should experience the discomfort of gore with their death spectacle), and just pop it when it’s time. Guaranteed to shut them off, mess is handled, suitable for a casket, and no suffering. They wouldn’t even have a chance to feel it.
And if the thought of putting a human down like cattle is disturbing to you, good. It should be, just like any other way we would keep somebody locked up waiting to be killed.