North Korea actually already makes millions (at least) of passable US currency. North Korea isn't impoverished because of lack of resources, but rather because the Kim regime would rather spend those resources on themselves.
Kim Jong Un isn't belligerent. The north koreans have a different playbook for foreign policy than you might be used to but its a playbook nonetheless. For their people the program is akin to something like the Apollo program in terms of national pride. Its also a dead man's switch effectively. The ruling family obviously wants to maintain their life of idyllic luxury and nuclear weapons and belligerent public addresses are a good way to make people second guess just steamrolling you over. In effect they are just playing a hand thats already dealt to continue their positioning.
Or you know they could become democratic and embrace market reforms so their people don't suffer so much oppression and poverty, but then the ruling class would have to give up its power. Then they could unite with SK. Nobody in the West is team rolling a democratic country.
Stop resisting and I'll stop choking you.