"Standard college harmony textbooks" don't even properly account for Mozart, Haydn or Beethoven, let alone these later composers. The shared structural framework that helps explain them all is thoroughbass, combined with a repertory of common patterns and moves (known as "schemata" in the recent music-theory literature) and styles of melodic elaboration that ultimately come from the improvisation-heavy performance practice of vocal music. (Conversely, melodic reduction would've been needed to understand complex melodies as elaborations of simpler ones. We now know that this was standard practice in the 18th-c. equivalent of solfège!) None of these things are explored, however cursorily, in the standard textbooks. Even counterpoint is not really developed properly - the way it would've been understood by successful 18th-c. musicians - and is often presented in a needlessly obscure and unfamiliar manner as a result.