Flute & sax embouchure changes over different ranges

I doubt very much that the clasical saxophone player does not make ANY adjustments to embouchure over the range of ppp low Bb to fff high C (altissimo) to fff low Bb to ppp altissimo C. I don't think the horn and the human body can actually be made to work that way.

I strongly suspect the "no adjustments" is actually intended to keep people from dropping their jaw enormously for low notes and biting like crazy for high notes, and rather to push them toward the actual subtle adjustments that are done by skilled players.

Certainly, using a 9* mouthpiece with 3.5 reeds is working way too hard unless you are trying to play in a rock band unamplified; and tonguing the saxophone to the roof of the mouth is an extremely unusual and non-standard method - everyone else touches the reed slightly. With a rig and technique like that, I'd never be able to get notes started and playing except with big tasteless honks and very little control of volume. (Not that I'm a virtuoso, but I've been playing saxophone for 40+ years in a wide range of settings.)

Why don't you try something like a #6 Link or #8 Meyer, and #2.5 reeds?

The "between the teeth tonguing" was a fad in flute instruction some years ago. I was urged by my teacher to try it, but I found it highly obtrusive and detrimental to double tonguing, so I dropped it.

Obviously it doesn't cross over to sax tonguing, where the long accepted standard method is to touch the tip of the reed lightly with the tongue (this gives you a wide variety of articulations depending on how firmly you tongue, whether you use tonguing to stop the air stream or allow it to continue, etc.)

As you re-engineer your sax tonguing, I would urge you to avoid "anchor tonguing" which is where one sticks the tip of tongue behind the lower front teeth and tongues to the reed with the middle of the tongue - considerably less agility than using the tip (more or less) to touch the reed.