This is brilliant. You have an astonishing mastery of harmony. I did a quick analysis of just the 1st minute (wish I had more time to do all of it!):
G | Bb | G | Bb / C
Love the C in the bassoon for color. This progression reminded me immediately of the progression in the new version of the 4th Dungeon (Angular's Tunnel) in the new Link's Awakening ( the I to bIII, also conveniently in G!)
G | Bb | G | Bb Again
G | Bb | G | E
Love this progression based on a minor 3rd from the tonic both descending and ascending when you are about to go into the next section
G | Gm | Dm7 | Fm7 --> Eb
What an absolutely brilliant way to go from G major to Eb major! Just starting all the minor chords in Eb to use this major - minor tension to enter Eb major while also just changing G to Gm. You definitely learned me something there!
Eb --> Gb --> Fm --> Bb --> (Flute melody ends with Eb D C D to E for perfect transition to...) C!
This meandering section seems to reuse the m3rd harmonic motif (bIII) again, but! then you seem to plant it firmly in Eb major with Fm and Bb... ONLY TO BRING IT TO THE CHORD A DESCENDING m3RD AGAIN, C major! And...
C | Eb | C | Fmaj7 (omit3)?
...now we're back at the main motif again but in the key up a 4th now! Wow. I'm just at awe at your craft in this piece. Bravo!
This is my own analysis of your piece, but, of course, I could be wrong with some stuff. Please correct me or fill stuff in if so!
My question to you thought is: how do you got about thinking about harmony when you're making a piece? Maybe that's too big of a question to answer here, but I would love to pick your brain on the subject! : D