We've done economic sanctions to greater than 90% of what can be done. Economic sanctions are wet trash. They have never stopped anyone from doing anything as far as I can tell. They also hurt the most vulnerable the most.
If there was a nuke launched at Berlin and that didn't result in NATO declaring war on Russia then there is no such thing as NATO.
I think the U.S. might make a limited nuclear response, targeting some remote area that has some sort of military or natural resource primary purpose. But on the heels of that would be full mobilization of NATO and declared war on Russia.
In reality, though, I think that if Putin edges too close to that cliff he gets done in from the inside. I don't think the launch happens because I think people very very close to him will prevent it and remove him and dance Putin's shackled self in front of NATO and say "we stopped nuclear holocaust, we will take care of Putin, just let us be the new rulers here and we'll play nice for now.
He's losing the war in Ukraine why on earth would he broaden it? Its simple saber rattling.