In particular, all of Seoul is in artillery range of the north

Rusty dusty equipment that probably won't even fire.. that offensive line is hardly a major threat

NK holds regular military exercises. And 152mm guns are tried and true; they aren't exactly the most finicky pieces of equipment.

Here's a massive artillery drill they held just a few days ago.

https://www.bloomberg.com/news/articles/2021-11-07/north-kor...

It doesn't matter if it is a 100 year old artillery piece, they aren't exactly complicated, just large breach loaded guns. The only thing modern guns and artillery have over old guns is accuracy, which doesn't really matter when you have enough of them firing down range at a massive target.

What's to stop China moving in with theirs? I mean after all they're friends and that's what happened during the lasy war...