How dangerous would moving to South Korea be at the moment?
No. Although North Korea might have had some suspicious nuclear activity, they will 99.9% NOT do anything dangerous. First of all, if they bomb South Korea (or attack), they will 1) affect their own selves with the radioactive materials from the nuclear weapons and 2) the whole world hates them, and they will soon get attacked by the US and a few other countries. Don’t change your plans, it’ll be fine. Relax. South Korea is a very fun and a good place for English-speakers willing to teach.