Unfortunately not familiar enough with Korean and haven’t tried to learn it myself. To be honest, while trying to learn Chinese, I think apps should only be supplemental anyway to help with things like proper pronunciation and speech. Actual textbooks have been significantly better for pretty much everything else. Both China and Taiwan have government crafted textbooks that target their language exams, so if something like that exists for Korean, that’s the route I’d recommend most strongly.

EDIT: It looks like it does exist and is called TOPIK, so there’s probably good textbooks that target those exams as a good baseline for the language.