I wouldn't be able to write with Bach on. The melodies demand too much attention to try to focus on another task. Lately I have been using vaporwave for music while writing. It is like library music from the 80's and 90's became self-ware. It is clever, pleasant but deliberately soulless, sometimes funny but sometimes trying too hard to be ironically hip. I don't like all of it, and I am almost ashamed to enjoy something that is so hipsterish. But, by god, it is good background music for writing.
Conceptually, I get what you are saying; but for my thesis, by the time I was writing it, I already had figured out most of what I was going to say. Developing the mathematical models required real thought. After that, crunching the numbers is rote, interpreting the results and determining which resulting info was relevant required real thought, putting the result into graphs is rote, written description of graphics is mostly rote, Abstract, Introduction, and Conclusion required real thought.
I go quiet for the "real thought" portions, but Brandenberg covered (most of) the rest, including on-going editing! So, the music is inextricably linked to the process of generating my thesis, but there were more than enough hours to go around!
But I agree. Brandenberg is not a good medium for deep thought. I also like to use it for cleaning house!