I agree with the two mistakes that were attributed to Descartes: (1) "...our knowledge of our mind's nature is more reliable than any other belief" and (2) "...his denial that other animals have any mental lives at all."
I don't think that leads to a total lack of free will, but our free will is part of a larger will. I wonder if that is what you are suggesting with the "need for an external system".
Here is an overview of Libet and his experiment: https://en.wikipedia.org/wiki/Benjamin_Libet