Are you familiar with MAD? It is not what happens when any two nuclear powers are in a conflict that escalates to nuclear use. It’s what happens when two superpowers with thousands of nukes exchange nuclear weapons after exhausting counterforce options and begin the grim work of headless countervalue exchanges from submarine platforms. If the US were to nuke Iran, who is going to press the suicide button on their behalf? There is no country that would guarantee countervalue strikes for Iran. Iran, if nuclear, would have one and only one leg of the triad, just enough to attack once and maybe not even that. North Korea’s security is backstopped by China’s deterrent, and their conventional artillery forces which could level Seoul. Their nukes are supportive of that, not sufficient in and of themselves. Iran has no true strategic partners, that is part of their defense doctrine.