I saw an interesting comment on Reddit. The question was “Where to start with jazz?”
Honestly, I’ve been fascinated and studied & played jazz for over 30 years and I would completely disagree with your statement “…the use of non-functional harmony…“. In my experience 99.99% of jazz harmony would fall into “functional harmony”. It’s just that with the frequent modulations, tritone substitutions, non-resolving ii-V’s, and other jazz idioms, it can be hard to make sense of it sometimes.
The best place to start is both listening to and learning jazz standards. Get a jazz fakebook like “The Real Book” or “The New Real Book” and learn 100+ jazz tunes. Woodshed on them for the next 5-10 years, memorize them, analyze them, absorb them. And you will start understand on an intuitive level, and then if you can take the extra effort to understand the music theory involved you’ll learn even more. Learn to play every chord, in every inversion, in any position, on any string sets, on the guitar.
Learn to play your basic scales (major, minor, harmonic minor, melodic minor, pentatonic, blues, whole-tone & diminished are a good start) and all the modes of each scale in every key & position on the neck, at any speed, using any rhythmic figure. Make sure you can play every basic triad or 7th chord arpeggio, anywhere on the fretboard. Realize that every triad arpeggio can also function as a 7th, 9th or 11th chord arpeggio, etc…same with 7th chords or arpeggios, etc…i.e. a Cmaj7 is also an Am9. Learn to understand that there is no such thing as a “right” or “wrong” note. There is only more or less dissonant notes.
Same thing with chords - any chord can be used at any time, if it sounds good. If you can’t create a melodic line using all 12 tones of the octave, something is wrong. Learn to let go of all your knowledge and use your ears to guide you. Then after like 10 or 20 or 30 years, you might actually find your unique artistic voice and become true musical artist. I’m about 30 years in, and still working on it.
(I broke up the block of text into paragraphs so that it’s easier to skim.)