Amazing, I was stuck in a windows-don't-open office around 10 years ago. Felt my concentration levels drop compared to home office. Just like you. I came to the exact same conclusions! :) Also purchased a sensor and confirmed the higher levels and noticed how keeping the door opened or closing it made a significant difference.

Even bought some plants, but they did not make any difference - they were too few for the space. To make a difference one would have to turn the office into a green jungle. And then, keep in mind plants produce CO2 during the night (when it is dark).

Btw it is Carbon di-Oxide. Not sure why you wrote it with 0 (zero).

After that I started paying attention what are the levels in the place where I live as well. Good luck! :)