Their one ace.
Most of the heavy artillery has Seoul in its crosshairs. Once the firing started, the amount of death rained down within a few hours would be staggering. It's the only viable threat they have, and everything else you said about their military is true.
We know where most of the artillery is. A first strike would allow us to control the tempo, not Kim. Put the THELs in place, send the ABLs aloft, hit the entire length of the Northern side of the DMZ with sustained MLRS fire. Begin taking out the command bunkers with JDAMs and cruise missiles. Then after a couple of weeks of nonstop bombardment, send in the Apaches and the armored spearheads. Allied casualties would be less than 100.