I think when you threaten nuclear war, you should ask for either:
1. White peace (which would have a higher chance of triggering) or
2. Unconditional surrender (which would have a lower chance of triggering), which would result in all of your war goals being pressed.
Either that, or make the enemy ai concede wargoals if you are the aggressor, and white peace when they are the aggressor.
Because currently "threatening" nuclear war is seldom good, because if you have nukes, you're likely stronger than your opponent and don't want just a boring white peace from which you gain nothing.