Side note: Even if the leadership were True Believers (which I agree they aren't), Juche doesn't seem like the kind of ideology that could really be used to justify expansion. I understand it to be pretty ethnocentric. At most it would support unifying Korea. But I don't see it trying to expand outside "the homeland". You might see import substitution, less-open borders, restrictions on foreigners, stuff like that. If Korea had some grand historical claims I could see it going irredentist, but I don't think those claims exist.

Their ambitions are curbed by their capacity, nothing more. The initial phase of NK existence was militant expansion, arrested only with crushing military defeat.

The broad framework of Communist states at the time was establishing world dominance through force and persuasion, although they couldn't agree on the exact pace and methods. NK was in this respect not an outlier, the Kim v1 likely seeing his role in the new world order less marginal than ruling half a peninsula. In particular, anti-Japan sentiment was and still is cultivated throughout society. In 2020 it can not be ascribed to realistic self-preservation interest.

And in either case, the very fact they do have ideology means a lot. Certain decisions are going to be made not due to strategic and personal interest but to an abstract rulebook not necessarily in sync with reality. Therein lies a huge danger not covered by assumptions of rational actor model.

NK is barely surviving. They don’t have enough ammunition and fuel for a full blown war. They don’t care about religion or doctrine. They are not trying to spread communism. They have no delusions they can take the Korean Peninsula.

Without anything motivating themselves besides just trying to survive, the motivation of everyone to fight is very low. That doesn’t make for a good dedicated army. Most NK soldiers would surrender as soon as they could.