For my own part, I suspect that consciousness is a strong indicator of free will.
If human agency were simply an integral part of a causal order, consciousness would not make sense and consciousness would, speaking from an evolutionary standpoint, be completely redundant. Why would nature evolve consciousness of the causal order, an order which inevitably determines every choice and act humans make, when being aware of the causal chain makes absolutely no difference to the outcome? It would be a complete waste of biological resources.
In sports, pinnacle athletes try to reduce conscious decision making to a minimum precisely because automatic responses built into the neurophysiology of the athlete are much more efficient in the demands of most sports. Reducing conscious decision making is the goal of most training regimes. So why would nature develop such a clumsy capacity when autonomic responses would be much more effective in the game of survival?
The answer, I suspect, is because consciousness is not a biophysical product at all, but an indicator of quite a different nature - personhood or personal identity that is only possible when an agent is “apart” from all else, including the causal order. You can’t have a subjective identity as a “person” without the capacity to originate thoughts, decisions and actions as novel to oneself and distinct from the external world.
Free will, I suspect is a necessary condition for consciousness and personal subjective identity. Without free will we would not have self identity precisely because there would be nothing, in terms of awareness, that would make personal identity a distinctly apprehended entity; nothing that would set subjective awareness apart from the causal order. We would have no sense of self awareness without free will.
I understand this is not an argument, but I don’t think it would be too difficult to develop a strong formal proof.