Hi @Henry Ng!
I'm very glad you enjoyed the piece! These are some high praises 🙂. I'll talk through my vision for the work - hopefully it will better explain a few of the features here.
Firstly, the overall texture I had in mind is that of an organ's. The initial French overture texture is in particular heavily inspired by BWV 552 ("St Anne" Prelude): heavy-handed voicing, ornamental, seventh chords, 2 & 4 suspensions everywhere. The following fugue remains in the organ texture - it achieves this by making the organ "pedal line" distinct. It offers an alternative view of the duet vs tutti sections as manualiter vs a pleno organo. It explains why the duets occur exclusively in the upper two voices.
I'd also love to claim that the fusion of ritornello and fugal form is innovative...But sadly the master himself has already done it in various major works. Some of the fugues from the Brandenburg Concertos and the Orchestral Suites have hints of ritornello form, but the most explicit examples can be found in his vocal music repertoire. See for example, the opening choruses of BWV 45 or 67 (for an even more complex example, see the opening Kyrie of the Mass in B minor). The overall scheme for these is:
1. Open with a complete ritornello theme (one that is very much like mine) on instruments. The ritornello theme itself must contains a "subject" which is passed a few times across the instruments.
2. The voices now enter procedurally in a fugal exposition using this subject.
3. Restatement of the complete ritornello theme in the dominant key, with the voices doubling instruments.
4. Instrumental interlude.
5. Free continuation. It could be a re-exposition, or wholly new material. Followed by,
6. One or two restatement of the complete ritornello theme. The last statement is in the tonic key and concludes the piece.
I think the most important thing to note from this scheme is that ritornello form forms the basis, not fugue. The presence of fugal material here serves no purpose other than to bridge the ritornello themes. This is exactly the principle I've taken in my overture, though my own scheme differs from these choruses: the opening ritornello theme is skipped entirely, and my approach to restatements of the ritornello theme is that of concerto rather than chorus - the ritornello theme is chopped up instead of being completely restated. Consequently, my focus is on the ritornello theme, and not on contrapuntal techniques such as stretto, augmentation, or inversion. Though stretti can be found, its purpose is to facilitate (either starting or concluding) imitative dialogue in the duet sections using material present already in the ritornello theme. This is why three-part stretto is not present (and why it wasn't even on my shopping list when I began architecting the work)!
The relevant example from literature would be BWV 1066 (Orchestral Suite No. 1). The subject can indeed be placed in stretto, but what Bach chose to do is quite puzzling. He uses stretto extremely sparingly, in seemingly unimportant places, and sometimes played by instruments which are not very audible (the second violins & violas). But if we choose to take the view that Bach's focus is on ritornello theme and not on fugue (and fugal techniques), then it actually makes sense. These stretti are placed right after the conclusion of the trios, and they serve as a way to procedurally, but quickly bring back the strings. Very clever!