Cool project! I'm a jazz musician / software engineer and have been building a similar personal project in my free time, writing a deterministic algorithm to perform harmonic analysis on popular standards, with the end goal of having an interface that could suggest chord substitutions to the user. I'm also looking at indexing all the tunes so the user can fuzzy search for tunes with similar chord progressions.
I'm curious to see how well you think the GPT solution works for you (I'm having trouble trying it because I'm so far back in the queue. Congrats on all the traffic!) My suspicion is that it would struggle to capture more nuanced substitutions, because it doesn't really have the underlying context (things like harmonic tendencies of individual notes and voice leading) to draw from but I've been surprised before. Hope I can try it next week!