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@PeterthePapercomPoser Hello Thank you. It makes sense what you are saying. Since an augmented 6th chord resolves well on the V, which would be F, over Bb. I think there are three points of view: Functionally, the solution you propose seems very possible to me. Structurally, it could be a Gbsus, or perhaps another nomenclature. But I think the key is that this bar runs through voice conduction, even if it cannot be clearly explained, or has more than one interpretation.

I think the chord is kinda functioning as a French 6th in Bb major. Despite the fact that it's missing an E on top to create the necessary augmented 6th, the function of the tritone substitution is still there since there is a Bb in the Violas. And the slip into the suspended 4th (Cb) and the addition of the 2nd (Ab) is a very individual and unique harmonic characteristic that makes sense with the voice leading between the two chords.