This discussion seems very interesting to me, also because I feel personally called into question... I won't bore you with my biography (!), I will only tell you that since I was a boy I began writing music, and for a long time I did so in a completely "instinctive" way, meaning somewhat by imitating the style of the piano pieces I was studying. Many years ago, I also attended a year of harmony (consonant only), but I do not have a good memory of it, partly because my relationship with the teacher was not exactly idyllic (he is a musician who is quite talented and well-prepared, but in my opinion too obsessed with rules). Then I decided to leave professional music studies to dedicate myself to something else (who knows, perhaps it was a grave mistake!), but in the following years I continued to compose, always basing it on known music (not just piano music), and simultaneously losing (unfortunately) my piano technique, which was already not impeccable. Only in recent years (by now... at a certain age!), I got tired of being ignorant and decided to look for a composition teacher to start studying more seriously. In these years, I have had the opportunity not only to do some harmony work that I had never known before, but also to experiment with chorales, orchestrations, piano romances similar to the old 4th-year exam, chorales, etc., and recently I have also started working on counterpoint. With this teacher, things are going very differently, not only because of his impressive ability to immerse himself with extreme ease into any style, but also because of his more relative view of the rules. To put it better: it is clear that the rules exist, but always "relative to a style". There is no such thing as an absolute error; there is only something that is "out of style". Therefore, for example, when harmonizing a chorale in the manner of Bach, it is appropriate to respect certain rules simply because that is how one obtains a result comparable to what Bach did, so it is right not to do something "because he wouldn't have done it". But the rules are not immutable; what seemed "forbidden" in a certain era was perfectly legitimate before or after, as happens in every evolving language.
Now, returning to the initial question (can one do everything alone?), I would answer "yes and no". If I could go back, I wouldn't wait so many years to start studying seriously, because effectively certain things can only be understood with a guide. For example, in many pieces written "in my youth," I faced the problem of finding a nice melody and then the right "chord progression" to harmonize it, somewhat like one does with songs; the harmonization was almost always for fundamental states, and only occasionally did I use inversions, often in a somewhat "strange" way, at least relative to the rules I WOULD LATER discover. Now I can say that I use certain resources of harmony more consciously. Therefore, from this point of view, a course of study has its utility. However, I would also like to speak in defense of "self-teaching": I am convinced that a composition diploma does not make a composer, just as knowledge of painting techniques does not make a painter. There must be a "natural instinct" in the search for melody and the development of forms—a natural instinct that is obviously better if it rests on good technique rather than fragile legs, but it cannot be absent. But there is a point from which, in my opinion, one cannot deviate: just as one cannot be a painter or a sculptor without knowing what others have done since antiquity, and just as one cannot be a writer while ignoring Manzoni or Balzac, it does not seem possible to me to compose without knowing much music, from various eras and for different instrumental groups. I don't mean only at a "technical" level, but precisely for the pleasure of listening, like someone immersing themselves in listening to a language to assimilate it and make it truly "their own". In conclusion, a teacher is needed, a clearly understood and assimilated technique is needed, but if one does not know music and it does not spring "from the heart," all the basses or chorales, the counterpoint or orchestration exercises that can be done in years of study do not "truly" teach art.
Forgive me for the long rambling. Greetings to you all.
Gino (Rome)