Obviously Mozart saved his greatness for his music
Some artists can only fully be appreciated by knowing about their lives and personality. Judging by these letters, Mozart is at the opposite end of the scale - his letters are more an obstacle to enjoyment of his music. I don't find his personality engaging or his thought interesting at all; not quite as bad as in the film Amadeus, but he comes across as vain, silly, superficial, place-seeking, childish - or, if you prefer, child-like. He rarely raises his thoughts from day-to-day matters. When he does, as in the 'Zoroastrian pamphlet', the result is merely cynical, commonplace wisecracking. I have to assume that when the editor, who seems unduly impressed by any hint of Mozart's esoteric interests, says of this pamphlet that 'Mozart has become a philosopher', he is indulging in a little German joke. Personally I don't believe that Mozart had any deep philosophical interest in Freemasonry, or anything else abstract. The reasons for his involvement were probably the same as Burns': to cock a snook at the class system and patrons; to hang out with like-minded cronies; and from a general vague sense of rebelliousness or (as Burns used to say) 'vive la bagatelle'.
There is certainly the occasional insight into his life, though you have to dredge through a lot of trivia to get it; very few into his work, which is discussed only in the most matter-of-fact terms. Clearly, he saved all the greatness of his soul for his music. Or maybe, if he had been a better writer - of broader interests, more verbal, more intellectual - he would have been a lesser composer.
One conclusion I definitely don't think we can draw, although he did die young, is that Mozart 'lived fast' - that he 'ran through life' more quickly than ordinary mortals, that the dissatisfaction he sometimes expresses towards the end was somehow final, and therefore his last works should be interpreted as expressing a tragic fatalism (or as being in any way a deliberate, definitive statement). Where only crumbs of Mozart's 'inner life' - supposing he had one - are available, any hint is seized on and exaggerated. One letter in particular, four years before his death, has been the basis of a view that he was in the process of finishing with life.
This is a misuse of hindsight. I don't read the letter in question as despairing or resigned, only sober and realistic; in fact since it was written to his father, who hadn't long to live, it may have been meant as consolatory. To recognise death as the 'goal' or destination of life is, on one level, stating the obvious; on another, it is wisdom; then again, it might simply mean a bout of low spirits. What it doesn't necessarily mean is giving up on life; surely all sensitive people feel at times that death would be a blessed release, but they keep living. If these letters show anything, it's that Mozart was not the type to be afflicted by existential angst; and I don't believe it was anything more than the start of a typical mid-life crisis. If he had lived longer he would no doubt have adjusted his perspective and learned to live differently, like the rest of us if we're wise (whether he would have continued to produce great music we can't know).
It is sad to see how he declined into poverty, jealousy, illness and death at such an early age, but it was all too possible in those days even for a genius. Unless you are amazed by the mere fact that Mozart was a human being who had to pay the rent - or not - like the rest of us, this is not worth bothering about. Any book about Mozart will cite the most important passages anyway. The production of the book is poor too, like bad photocopying.