It depends on how the war progress.
Artillery heavy wars are slow (again, just look at how Russia maneuver their troop), and area focus, because your artillery would need to be in the vicinity of where you want to attack from. As such, it is very prone to airstrike. Which is what the South are very good at.
The 20K artillery may be an impressive number, but it wouldn't actually work because you either need to coordinate your attack with infantry (So basically you drop PGM round in as your infantry advance) or you want to saturate your target with artillery and then move your troop in, North lack the capability to do the first thing (Otherwise they wouldn't need 20k artillery piece) and it would be a nasty surprise for the North Korean artillery if the South can enforce a blanketed Aerial Exclusion Zone.
As for they will level ROK air force on the ground.....Well, South Korea lack the depth, but we aren't talking about the entire South Korea is 40km in length, they can't shoot over their range, South Korea end-to-end is about 400km long, there are tons of airbase is going to be untouched by North Korean artillery. You are talking about 40km range for Artillery and <80km range for MLRS
Plus, some of these airbases are US Air Base, they either have to attack them and then invite angry American with 3 Carrier Battlegroup to fight against them, or they will have to left them alone and let the South use them to fight back.......
North have no chances overrun the south as long as the ROKAF is active, and they can only remotely pushes that back if the Chinese decided to make it their own problem.
