Now that we have the smart munitions, the fact that pieces can fire a few rounds then be pulled back in a cave is somewhat less critical. Even if there are blast doors if those are trashed and the piece is stuck inside the cave that works well. The problem is there are a lot of these. Not all of the positions are known, and I would bet some positions are dummy/empty. How many of the smart munitions that can be steered in to a cave mouth do the US and ROK have - enough to take all of these out? How many days will it take to do so even with "unlimited" munitions? Remember you'll need to suppress air defense, you WILL lose some aircraft, and you can't send all attack aircraft to do this there will be other missions that need to be performed - attack artillery or support troops keeping the NKPA from actually getting to Seoul will be an issue. The key issue is how long will it take to trash enough of the artillery so that attacks on the Seoul metro area stop or become merely annoying, although random artillery fire in to a metro area of 15-20 million is not conducive to preventing panic.
To make matters even worse, I would expect the DPRK to mix in chemical shells with the HE ones. When the artillery starts folks will go to shelters, basements, the subway system etc. If 1:4 or 1:5 shells are nerve gas or mustard, which are heavier than air those underground shelters will become abbatoirs. As folks on this board should know, the only defense against nerve agents is full suits (MOPP-4 in US terms), and I doubt everyone in Seoul has a gas mask let alone a suit. Nice choice, stay underground and risk be poisoned or come up and risk being eviscerated. The point of this is to create mass panic, force the ROK/US forces to deal with refugees in all directions clogging the roads (and providing cover for NK SOF) and the NKs might believe that the ROK population would be too weak to stand up to this and would demand a surrender (wishful thinking but...).