OK, would you like an autodidact's viewpoint? I started teaching myself composition over 40 years ago. For becoming adept in music theory, @user18957 is entirely correct: there isn't a king's road. Even growing up on King's Road didn't help. (Joke, but that really was the name of the street on which we lived.)
There isn't going to be a systematic way of going about it. You need to be driven by a burning curiosity - when you research in one area, you'll necessarily follow up on matters that pique your interest that you run across while doing so... any matters. You aren't going to stop with Piston's Harmony; you will lay hands on historical books like Rameau's Traité de l'harmonie and exploratory ones like Schoenberg's Harmonielehre. You may pick up things like Hindemith's Unterweisung im Tonsatz and George Perle's Twelve-tone Tonality. You may well pick up books on atonality and/or the twelve-tone method by René Liebowitz and George Perle. You may pick up Messiaen's Technique de mon langage musical.
You won't just stop at Piston's Counterpoint: you'll get Fux's book and any others that catch your fancy. You might pick up Gedalge on fugue. You will pick up a score of the 48.
You will probably pick up collected essays by people like Schoenberg (Style and Idea), Roger Sessions and Béla Bartók. You might pick up Charles Ives' Essays Before a Sonata and Ferruccio Busoni's Sketch of a New Aesthetic of Music. You will pick up books that analyse the works of a period or of a composer, books like Jarman or Perle's books on Alban Berg, like Charles Rosen's books on the classical style and the romantic period, and so forth. I have a book in my library analysing Arnold Schoenberg's music before Verklärte Nacht - the contents of your library will necessarily differ according to your tastes and interests, but you should have room for books that are equally specific.
And you will collect scores and read them. You will, in fact, have scores coming out the wazoo. (IMSLP, IMSLP, IMSLP! You are so lucky to have resources like this available. I had to buy all my scores as a young man, and they weren't cheap.)
Now, I have or have had all of the books I have mentioned in my library, and I've barely scratched the surface. Your library will necessarily differ as you follow your nose. The point, though, is that you must follow your nose. You cannot become a "guru" without arriving at a deep understanding and viewpoint that is entirely your own, and you can't achieve something that is entirely your own without following your own interests where they take you, revising your views as you go. The process never stops.
And you know something? In another 30 or so years, you will know less with certainty than you do now, you will have more questions without answers, and that's how you will know that you are finally starting to understand music.