The only reason South Korea is as advanced as they are is because of the massive amount of funding they receive from the U.S. government. It's given them the opportunity to invest heavily into their own infrastructure, pulling them out of the dirt and into the developed world. Before the split the entire peninsula was just like how you described: poor peasants, lackluster infrastructure and rampant oppression.
The only Asian country that really has avoided all of that without assistance is Japan. They have a very high standard living, one of of the best in the world. The government is even trying to invest more in the rural areas of the country in order to help keep them running and connected with the more urban areas.
Unfortunately they too have their own problems with an aging population and a low birth rate, which has led to a decline in various skilled trades and more blue collar working positions. On top of that they have a
very fucked up work culture, which hasn't helped the already high suicide rate. Japan obviously has a long history of overcoming their problems and putting themselves back on track, so it's not impossible for them to fix these ones. However, it's going to take a lot of work over the next few decades.