Poultry Council does show CO2 as an "approved" method of euthasia if you want to get picky.
I don't advocate for home methods of ether, co2, vinegar/baking soda, etc. BUT when you get down to it, it is the caretaker's decision to determine the method.
Quick is always my goal for any of my animals. So no matter how much my heart hurts afterward, I try to do what's best/quickest for them.
Again...I ask you
@Fishkeeper how many chicks/chickens and/or quail have you actually held in your own 2 hands and culled yourself?