It would be interesting to see a database from a different historical period of American popular music, as I would surmise the frequency of both notes and chords might be very different in, say, the minstrel period (1840s-50s), the parlor period that followed, or the ragtime and early trad jazz period in the early 20th century. In the last two, for instance, the VI and II major chords were integral in a huge proportion of the song base, way above the 2% and 6% in the modern one. It would be fascinating and useful to have something along the lines of a music version of the Google Ngram, that could show the rise and fall of relative notes and keys (or perhaps more, like phrases, tempos) as both styles and instruments came into vogue. Anything like that underway?
And for another subject entirely, way back in 1972 I made up a little graphic songwriter’s aid for choosing harmonically close or distant chords and notes in all keys (and can help transpose as well) that provides a fun but very simple thumbnail of many of the issues that Hook Theory addresses. Called “The Harmony Crystal” you can take a look, download and have fun with it at http://www.astrococktail.com/harmonycrystal.html with rudimentary instructions, even some astrological connections. Certainly used it myself when I was a salaried songwriter for MCA, CBS, and Buddha/Kama Rippa publishing outfits at that time, had to turn out stuff whether the muse appeared that week or not…!