What preparations is the ROK making against North Korea's nuclear weapons? South Korea's preparations against North Korea's provocations and missile tests should be flawless.
From what I know, South Korea has established a firm defense posture through the ROK-US combined operations. Moreover, it's been told that the ROK, itself, has the capability to deter North Korea's missile attacks.
South Korea has the capability to detect and intercept North Korea's missiles through advanced radar technology.
Of course even if they have advanced radar technology, not all missiles would not be intercepted. Some may inevitably lead to heavy damages. However, I believe that North Korea will not have superiority over South Korea.
If North Korea attacks with missiles, North Korea will receive counterattacks, which will cripple North Korea. The best option on the table for North Korea is to refrain from any further provocations.