Not the person you're responding to, but I think some people can still think of free will as possible in a deterministic universe without thinking that mental processes are somehow not in the mix with everything else.

I do think there are distinct sub systems to a deterministic universe. All things are equally determined and definite in their eventuality, buuut, not all things are of the same magnitude or complexity. The marbles on a track of molecules in a creek are not as sophisticated as the rube goldberging of a mind. To that notion, the more aware of a deterministic state a sub system of a determiniatic reality is, the more it can sort of adjust. SORT OF. We can divert creeks, and hoping for an at least pleasant way along the tracks for those now and in the future, thinking beings can seek a less painful average for their existence. After all, if the hill top of pain is percieved as negative, it only makes sense that all thinking beings would drain downhill towards the less painful reservoir below right?

Definitions are allowed to, and just sorta do, change, and choice/free will might be slightly different in the context of the universe than we initially thought. Like, our galaxy is not milk, but it's still called a galaxy. . .a word derived from thw greek word for milk. People don't still argue whether of not the stars are milk, maybe we need to stop arguing about the stagnant definition of free will not being compatible with a deterministic universe :/