Even very plausible low thousands of artillery pieces can put out enough dakka to slag anything looking even remotely like an air defense radar or missile launchers.
Air defense radars, missile launchers and even Seoul besides northern outskirts aren't generally in a range of tube artillery, which for the 152mm soviet types utilized by NK is around 20km.
And you're not putting your guns literally at the frontline - you need them more back so they don't get counterbatteried (or just bombed, which would be more likely in case of US involvement) instantly.
And you're not putting your guns literally at the frontline - you need them more back so they don't get counterbatteried (or just bombed, which would be more likely in case of US involvement) instantly.