In your thought experiment, you basically say, “Let’s model the universe with the assumption that there’s no free will” and use that assumption to show that there’s no free will. If you assume that the universe functions like a computer, you’ll get that result, but I need more proof to accept that the universe functions like a computer. I highly doubt that it does. If this was the seventeenth century, the argument would be that the universe functions like clockwork. That was a decent approximation at the time, but clearly not true.

I can’t say that there IS free will, but I think trying to make the determination that there isn’t with today’s level of understanding is premature. Since free will (whether real or illusory) is either an effect of consciousness or at least deeply tied to consciousness, I think we need a coherent theory of consciousness before we rule out free will. We don’t have that theory. That’s why it’s called The Hard Problem of Consciousness.