Ask a musician about Beethoven’s choral writing in the 9th symphony and elsewhere. He had no feel for the human voice. None. His one opera Fidelio has great moments but is not vocally adroit. The Choral Fantasy is a minor work that is virtually never performed. ‘The Magic Flute’ pops up on lists of the greatest operas ever written. Mozart actually revolutionized opera and the concerto. He pretty much invented the piano concerto as it came to be understood. There were no stand alone overtures or programmatic orchestral pieces in Mozart’s time so it’s useless to look for a corollary to stuff like the Egmont and the Consecration of the House. Mozart also gave up writing violin concertii in his teens.
You seem to be putting eras up against each other as opposed to composers. In the 18th century the qualities looked for in music were balance, clarity of line and emotional restraint. Those are the qualities Mozart exhibits. He was also the most radical composer of the 18th century...harmonically much more advanced than Haydn. He’s a utopian composer. His music has an otherworldly beauty that Beethoven hinted at occasionally.
And how can you say on one hand that he excels at those qualities you name as hallmarks of 18th-century classical music -- balance, clarity, and restraint -- and then on the other hand proclaim him "the most radical composer" of the era?
I agree he was more harmonically advanced than Haydn, but that doesn't say much. Haydn is indeed marked by the virtues you listed; his precision is unmatched in the canon. But in deviating from his teacher, Mozart BROKE those traditions; he didn't embody them.
Mozart's music is fun. It's entertaining. It's self-consciously experimental and even challenging at times. But Beethoven captures the music of the spheres. The power and might behind his authoritative, even Teutonic themes are approached only by Wagner and Bach.
Music to Mozart was a toy. Music to Beethoven was an art.
Also keep in mind that Mozart died before turning 36. He did not live long enough to fully develop at a symphonist composer. Only the last 6 symphonies were composed in his maturity...in his time symphonies were mainly a way of making a splash in a new town. He was an operatic composer who wrote piano concertii to make a living, piano sonatas for his students and chamber music for his friends.