local breakthroughs on the major routes towards Seoul—their first operational objective. Playing devil's advocate: How do we know Seoul is the operational objective? It seems obvious (capital city located near the border) but what if DPRK went for something not so obvious? i.e. "Attack him where he is unprepared, appear where you are not expected" (classic Sun/Mao approach) Also, what if Pyongyang tries to draw in a…
There just isn't that much else in the border zone that would make for an attractive target. East of Seoul is Gangwon province, which is relatively mountainous and sparsely populated. Mountains are hard for an army to move through, especially an army with tanks, and the sparse population (around 1.5 million people, compared to 10 million in Seoul itself and 15 million more elsewhere in the capital region) means that there aren't a lot of targets of high economic value there.
About the only military value of a move towards Gangwon instead of Seoul would be strategic surprise -- an attempt to shock the allied forces by appearing from a direction where they aren't expected, the way the Germans shocked the French and British in 1940 by driving into France through the (generally considered impassable to tanks) Ardennes Forest instead of via the open plains of Belgium (see https://en.wikipedia.org/wiki/Battle_of_Sedan_(1940)). But that kind of surprise would be much harder to pull off in an age of high-resolution satellite reconnaissance and instant electronic communication, and it would mean forgoing the hammer blow on South Korean morale that the loss or destruction of Seoul would cause. So it would be quixotic at best.