I'm not a millitary strategist in any way shpae or form, but my understanding is that with Seoul within the reach of fortified artillery, etc., in NK, the north gets to play terrorist. They have 10M+ people they can threaten but know that one shell into Seoul is all out war.
While the logistics would be massive, I would think that SK could return pressure without firing a shot by announcing an orderly evacuation of Seoul to southern areas of the country. Just the announcement would send chills, actual movements would likely be seen as dead ass serious as mobilizing troops and equipment on the border, but would nopt be offensive. Any offensive action by NK would be seen a their desire to effectively hold civillians hostage.
I'm sure there are smarter people than on this, and there certainly would be economic impacts to SK and globally. But a firm response from the world saying we will float / forgive resources during this critical period would be one way of providing a unified response with real teeth, and make the cheese a little more binding for NK.
On the other hand, SK knows more than I do and might have a plan in place to knock out the threat with limited damage. I wouldn't put it past them.
They won’t evacuate south. More likely north figuring they are going to die anyway.