Reddit mentions of Pieces of Tradition: An Analysis of Contemporary Tonal Music (Oxford Studies in Music Theory)

>I'm trying to put together a plan of materials to go through with the intention of becoming an "expert" (very adept, lets say graduate level) in theory over the next several years.

So, at minimum, you'll need to know tonal (Schenkerian) analysis and post-tonal analysis. The fourth edition of Joseph Straus' Introduction to Post-Tonal Theory is good for post-tonal. My Schenkerian class didn't use a text, but Cadwallader and Gagne seems to be a thing now.

At the graduate level, studies are motivated by the student's research interests. It sounds like you are interested in what Dmitri Tymoczko calls "the extended common practice."

For breadth, read journals and publications. MTO is free, Spectrum is a big one, and so it JMT. Here are the last five recipients of the Wallace Berry Award (and you can read more here):

Steven Vande Mooretele - The Romantic Overture and Musical Form from Rossini to Wagner

Daniel Harrison - Pieces of Tradition: An Analysis of Contemporary Tonal Music

Ruth DeFord - Tactus, Mensuration, and Rhythm in Renaissance Music

Jack Boss - Schoenberg's Twelve-Tone Music: Symmetry and the Musical Idea

Elizabeth Hellmuth Margulis - On Repeat: How Music Plays the Mind

Given your interests, I'd definitely read the Daniel Harrison book.

/u/Jay13232 mentioned Persichetti. If you're going to read it, do so after you get a handle on set theory (from Straus). It's a good book, but our modern methodology is better for describing that repertoire in my opinion. Persichetti and Hindemith are like whacking nails into a board with a wrench (using ideas appropriated from tonality to describe music that doesn't follow those principles). Allen Forte, John Rahn, Robert Morris, and Howard Hanson gave us a proper set of hammers.