From what I understand, since your layover is less than 24 hours you shouldn't need a transit visa for Thailand. The guidebook says Don Mueang and Suvarnabhumi airports are about an hour drive from each other with traffic, so that timing should work out okay. Have you looked into airport transfer options like the Airport Rail Link? Might be less hassle than a taxi. Excited for your Korea trip! Have you been there before or exploring for the first time? May this tourism guide help you too https://gowithguide.com/blog/south-korea-tourism-statistics-2023-all-you-need-to-know-5254 Safe travels!