Free Will vs Determinism

First of all, it's a false dichotomy.

The assertion that "free will" is a subjective illusion (which I make) does not necessarily presume a "deterministic" universe.

There is a third possibility...and that is the universe is at least partly random.

It's too simplistic to say this lack-of-determinism comes from, say, the effects of distributed free wills.

We see lack-of-determinism as a fundamental feature of nature in quantum physics.

Things don't have a state, until they are measured.  That means no set of knowable variables about the universe is going to tell us the state of that thing.  All we can say about it, before measurement, is in terms of probabilities.

 Now suppose quantum effects ultimately affect decision making processes in the human brain.   That's hardly "freedom" if your decisions themselves are not being made by you but by random processes in your brain.

The ultimate thing is, there is no you.  You are a story you tell yourself, nothing more.  Your decisions are made by the sum of all things that have come together in you, your genes, and every single moment of your life.  That's what makes the decisions.  Then, you tell yourself a story about it after the fact, and later.  You say, "I did this because of that."  There may be some truth to it.  But it's just as story about You rather than the set of things which actually did it beforehand.