With a full scale nuclear exchange we would have a nuclear winter for a the first few years (when the radiation would be the highest). This is an interesting read that looks at actual US government procedures following and exchange. It's a bit dated, but was written during the height of the cold war. https://www.amazon.com/Day-After-World-War-III/dp/0670258806
On the specific question of how to keep policymakers aware of the gravity of the nuclear decisions they could make, it notes there was a proposal floated at one point (half-jokingly, but only half) that the launch codes for American nuclear missiles, instead of being carried around in a briefcase, should instead be surgically implanted under the skin of one of the President's aides. That aide would then be responsible for carrying a butcher knife on their person at all times.
Why? That way, for the President to be able to launch the missiles and kill millions of people, he'd have to be willing to kill one person with his own hands first.
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