Regardless by the time Germany accepted that they were facing an existential threat, they couldn't have used Chemical weapons to completely decimate their enemy the way a losing country could with ICBMS (both land based and on submarines).
They had no way to reach America, or even Britain at the end of the war (with enough munitions), so chemical attacks would have been pointless by that time. If Hitler could have pressed a button completely destroy America, Britain, and Russia when he was in his bunker in Berlin, I'm willing to bet he would have.
https://en.wikipedia.org/wiki/Battle_of_Changde#Use_of_chemi...
https://en.wikipedia.org/wiki/Japan_and_weapons_of_mass_dest...
Sure they did. They were just used on civilians instead of regular combatants. What was Zyklon-B, chopped liver?