Bach is exactly on the cusp between linear counterpoint, which didn't use functional harmony, and later classical music, which did.
Bach did not agree with Rameau's take on harmony. You can see this in CPE Bach's Essay on the True Art of Playing Keyboard Instruments, which was written by his son and is a decent distillation of Bach Sr's approach.
There are vertical arrangements that can be recognised as chords and modulations, but there's also a lot of horizontal shaping in the lines between them.
This is how you get fugues, canons, linear inversions, retrogrades, and so on - none of which can be analysed using simple chord theory.
There is far, far more going on than the occasional use of what we'd now call a deceptive cadence.
Bach did not agree with Rameau's take on harmony. You can see this in CPE Bach's Essay on the True Art of Playing Keyboard Instruments, which was written by his son and is a decent distillation of Bach Sr's approach.
There are vertical arrangements that can be recognised as chords and modulations, but there's also a lot of horizontal shaping in the lines between them.
This is how you get fugues, canons, linear inversions, retrogrades, and so on - none of which can be analysed using simple chord theory.
There is far, far more going on than the occasional use of what we'd now call a deceptive cadence.