South Korea alone outclasses the North by at least an order of magnitude.
A couple iffy nukes won't make a difference.
North Korean artillery could flatten Seoul in a couple hours as surely as a few nukes, but then North Korea would cease to exist. Utterly.
Kim has a pretty cushy thing going, I'm sure he isn't looking to end up dead.
You need to be reminded how U.S. couldn't win Korean war, even before NK had nukes.