Isn’t the answer, then, to just give North Korea 10x as long as a “managed” colony before unification?
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Or, alternately, given that literally everything in NK would need to be rebuilt at some point... rather than "swallowing the elephant whole", they could take incremental bites: annex a few miles of North Korea into themselves; revitalize/gentrify that part; repeat. Slowly, over 50 years.
(How? At the end of the war, they'd do a census, figuring out where every NK citizen lived as of the end of the war. Then, for each annexation, anyone already living in the annexed area as of the end of the war, would automatically become a South Korean citizen. Anyone who moved to the area after the end of the war would not, and would instead be required to vacate northward, with the SK government compensating them for the [very low value of the] home they're being displaced from, per eminent domain.)
Of course, any NK citizen would be free to apply at any time for a South Korean visa — just like a citizen of any other country can. They wouldn't be especially prioritized.
And an NK citizen would also be equally free to apply for a visa to move to literally anywhere else — possibly as a refugee. SK would probably encourage this and even assist with it.