I think that you are right in that Seoul would be an absolute tragedy. That said, if (and I really mean if) the situation could be abstracted to a choice between an American city or Seoul getting bombed, then for an those charged with protecting American national interests, the choice would be a no brainer.

Now back to that if.

The current regime in NK is rational and interested in self-preservation above most else. That is generally agreed upon. So it is not likely they would attack first. So Seoul vs American City is not probably a real choice.

That said, they have no wish to integrate into the international order, frequently make threats of mass destruction, and seem likely o try to use nuke capability/threats to make demands on their neighbors. They are flying missiles over Japan. Then there is the fact that the regime is genuinely evil to their people, which makes political upheaval more likely given any weakness, sudden events, etc. And frankly, I do not like the threat of nuclear exchanges that can reach my family and friends from an actor as irresponsible, immoral, and backasswards as NK. It is already a tragedy that the largest countries have nukes, but proliferation is very dangerous.