The Magic Montain is about the maturation of a dumb adolescent..
Death in Venice is an excetion just like Buddenbrooks is, but in another direction. The book is not about this young guy but about the combination of death, unfulfilled desire and creativity within the artist's mind. Gustav von Aschenbach is the main protagonist there, not Tadzio. Tadzio is just someone that represents the platonic idea of beauty.
And we should keep in mind that Mann wrote again very diffent works after he received the Nobel Price, he wrote
Joseph and His Brothers about a mythological topic for 16 years;
Lotte in Weimar: The Beloved Returns, 1939, as a response to Goethe's 'The Sorrows of Young Werther'; and
Doctor Faustus, 1946, as a late novel about art, syphilis and modern music.
Just like with Nietzsche, Goethe or Picasso there are very diffent stages within the works of Thomas Mann with their own specific topics and artistic styles. There is no single work that represents Thomas Mann as an artist.