A lot of these numbers thrown around for DPRK artillery and its effects are greatly exaggerated.
Not all of it is across the border from Seoul, and only some of it can reach the northern extremes of Seoul, and then only a fraction of it can actually reach all of the vast Seoul metropolis.
And cities and human beings are far more survivable than you would think. Seoul has solidly-built buildings and people aren't going to be standing around gawking on the street after the first shells fall.
It will be bad but take with a giant boulder of salt some of the extreme predictions.
There are less than 20 bridges in Seoul that will get you to the south side of the Han. You have to assume that at least some of those will be targeted by FA and/or saboteurs right at the onset of hostilities. If you've ever tried to leave Seoul at the start of Chusok, the major arteries are parking lots, and that's a holiday without any mass panic.
If the NORKs drop chem rounds, it would be a slaughter on a scale that would make the firebombings of Tokyo or Dresden pale in comparison.