Update: I made the train with time to spare! My flight was early so I had a head start, but I would've made it comfortably regardless. Deplaning and immigration were a breeze; so much so that I beat my checked bag to the carousel. Customs took 15 seconds, SIM card pickup five minutes, and my preprinted KTX ticket turned out to be an actual, ready-to-use ticket. I had so much time left over that I ate dinner, got cash, and purchased and loaded a T-money card.
