If I understood you correctly, you are looking for
• a large briefcase, office/ professional looking (not looking like a typical backpack),
• that can hold your large laptop and other items in a structured way (some dividers inside rather than just an open bag),
• and you are okay with leather or cloth but not hard plastic.

I'm sure I missed some of the points you listed but I'm trying to focus on a picture of what you WANT, rather than confusing the issue with an additional list of Don't Wants.

I have something that actually fits all of the above.
The only negative is it's leather and is very well made, so there would be a substantial weight penalty.
I ride with it as a backpack on my motorcycle and the motorcycle doesn't seem to mind.

When I am the engine, I'm not sure it would be as fun. No doubt it would be hot after a while unless you were dressed for a workout.
On the other hand, the straps (shoulder straps are actually converted cross-body strap) could be made to wrap around the rear rack.
Then you would have a very large pannier on one side. I don't know how that would affect the ride/ steering with a lot of weight on one side like that. Maybe one could get used to it. Using it as a pannier would certainly keep it off your back and that would no doubt keep you cooler.

Last time I used it was on a 4 day business trip. It was my carry on and only luggage. It fits under the seat in front or over head of course.

Is that sort of what you want?