I currently know how the circle of fifths works and how different modes work.
Most YT content regarding music theory seems to either cover only the really basic stuff or focus on applications of music theory like special cool chord progressions instead of explaining how stuff like harmony actually works. Can yall guide me toward some easily accessible content that will help me learn on a conceptual level what is next, after keys and modes?
totally depends on what your ultimate goal is.
independent from that, i recommend the highly customizable exercises at musictheory.net - knowing how stuff works is good, practicing it until you recognize it instantly in sheet music and/or on the keyboard is a whole other beast.