You can only book a flight to Jeju as a resident and there are special rules to follow. To go literally anywhere else - you must be funneled through ICN...from there it's a bus to your town OR you take a bus for arriving passengers only to Gwangmyeong where then you take KTX (again a special car) for arriving passengers to other areas. Even then, as a resident, the rules very much depend on your local health office's system. As a tourist no residency, absolutely you quarantine in Seoul area and don't move an inch past ICN.

Don't expect the Busan flights to return for ICN transit passengers for a LOOOOOOOOOOOOOOOOOOONG TIME.