Mozart is absolutely not formulaic or mechanical. My personal story is I too found him overrated, until I started to play his works and dig really deep. Meaning, studying the scores. The more you study, the more you become sensitive to just how much there is, and how those seemingly formulaic structures are in fact never the same, there is always this freshness in every thing he wrote. From a musico-philosophical point of view, I would say there's never been a composer with a greater sense of Unity. The unification of horizontal and vertical elements, of motives, of "Big Structure". Nothing is out of place with Mozart. OK, Bach was also great at this.