hmmm i've actually been pretty impressed with apple music's selection. it's mostly the really popluar OSTs. i've been able to find: strong woman do bong soon, goblin, cheese in the trap, moon lovers, weightlifting fairy kim bok joo, w, kill me, heal me, i hear your voice and even doctor stranger. needs a little bit of creative searching, but it still works. by the way, i'm in the US.
also, i've had similar luck with spotify. they are usually organized in published playlists that you can follow. they don't usually include the instrumental tracks. if you have any questions, feel free to ask me! happy to try to help!