where would I learn a taxonomy of such motifs? I trained for classical piano as a kid, and I'm slowly trying to learn jazz piano as I ease back into it! I'd love to learn more about these types of harmonic ideas and motifs as I get back to playing

I think the word "taxonomy" is a really good way to look at music theory. Really, the study is about giving names and notation to things in music so we can write and talk about it at all.

There's an interesting idea though that there could be a deeper why specific sounds and their combinations are the things we create names for, but that's an intersection of cognition/physics/culture that goes beyond elementary music theory.