Planning guides cover the decisions you make at home, before anything is booked. Entry paperwork comes first and catches people out, because being exempt from the K-ETA is what makes the e-Arrival Card your responsibility rather than removing the admin altogether.
After that the questions are timing, length and money. October and April are the reliable weather windows. Five days covers Seoul, and the KTX makes a second city cost a morning rather than a day. One transit card works nationwide, even though the fares and operators change between cities.
Every figure in these guides carries a source and the date it was last checked, because entry rules and fares in Korea change more often than the guides describing them get updated.