By an interesting coincidence, SK’s K9 self-propelled gun has a range of 40 KM. A significant portion of the dug-in NK stuff will reach out to 60, but it’s random fire at that range.
SK also has some interesting missile capabilities.
And South Korean artillery is mobile, has accurate counterbattery radar, and can be near the border but not on it.
If North Korea wanted to “decimate Seoul” they’d have to put everything in a very small basket close to the DMZ, and that’s assuming that the artillery systems that North Korea has can achieve their maximum theoretical range.
Seoul faces a much greater threat from FROG and SCUD unguided ballistic missiles, IMHO.
South Korea has had years to worry about this problem, you can bet they’ve done something about it.