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.
The approach to music as an art and a heavenly endeavor was new to Beethoven’s time and Mozart simply would not think that way. No one of his time would. It was a craft to him. The word Art or Artist never occurs in Mozart’s letters. You’re projecting attitudes backwards into a time when they did not exist. Mozart was radical in his harmonic freedom but his music certainly does exhibit the core qualities of the era. Mozart would never start a symphonic work with a terse motif like the opening of Beethoven’s 5th. To Mozart that would be ugly music. A better question would be who is superior to Mozart in the 2nd half of the 18th century? You seem to have a condescending attitude to the music of that time.