I would add that between them, the South Koreans and the Japanese should be able to field at least 500 combat aircraft.
Given that in the event of major hostilities, the defense of Seoul would have to be a very high priority, the immediate area to the north of Seoul could be rather crowded!
Assuming some of the Norks’ planes would be taken out pretty quickly by cruise missiles and such, I’ll assume 500 planes would establish air superiority, leaving 1000 planes (plus a number of attack helicopters and such, PLUS fairly formidable South Korean* and US artillery and missiles) available to clobber any Nork artillery or missile launchers poking their heads out to bombard Seoul, or attacking Nork ground forces.
How long does it take 1000 planes to take out 10,000 artillery and launchers? Hmmm...
*Note that S. Korea spends 3 to 4 times as much on defense as does N. Korea. Of course some of that is spent on pricey US weapons, but, most of those are darn good systems.
Problem is once 10,000 artillery shells are in the air. They are going to hit Seoul. They have to be taken out before they are fired.
How long does it take 1000 planes to take out 10,000 artillery and launchers? Hmmm...
https://www.youtube.com/watch?v=bg9uoI8RQKc