I'm not going deep into Jazz, but if I'm playing a song with something like I vi ii V, then I'm going to likely color those chords with the 7th (at the minimum)

I'm not going deep into Jazz, but if I'm playing a song with something like I vi ii V, then I'm going to likely color those chords with the 7th (at the minimum) and likely additional extensions. I picked something like I69 not because it has function different from I, but because a 6/9 chord is a common choice for a Jazz musician to play as a I chord. Or, at least, it's one of my first picks. And if you're playing I9 -> IV9, my question is whether the I IV is just comping on I or whether it's harmonic movement. And I will say that while there is no requirement to use functional harmony in Jazz, there is a massive repertoire of Jazz that uses functional harmony, or at least uses harmony where you can get insight by analyzing it functionally. For example, I might analyze something as "bV7/V I" in a jazz song and that is both very functional and very jazzy, and in a classical piece I might see "N6 V7 I" and while N6 and bV7/V are similar chords, using the chord with dominant function is, stylistically, a jazz idiom.