I'd like to do the same but wonder how to fulfill K-ETA and special quarantine report requirements for a Korean address, zip code and local phone number. Aren't both these needed when leaving the airport?

When I tried 'test' filling them online, I could not bypass the Korea address requirement. The systems are also quirky - I couldn't paste a Seoul hostel's address in a test application.

It also looks like transit tours have been cancelled due to Covid.