It is fair, I think, to set your own boundaries with how close to choking you are willing to get - consent requires BOTH to enthusiastically consent, not just the sub.

I think a safer scenario involves some playing along from the sub. A firm non-choking grip just below the throat to signal to them to hold their breath - that way they can actually breathe the entire time and reflexes can kick in if they need to, but you are strongly encouraging them to play along with the pre-established signal instead of actually restricting airflow. To someone deep enough in subspace that can feel just as intense as an actual choke without the risk.