One way to test for causation is to see which one happens first. If free will (at least as defined above) is true, we expect that some subjective experiences should precede the corresponding physical change in the brain. If free will is false, then we should expect subjective experiences to be simultaneous with or come after the corresponding brain change.

Only if you assume that "free will" implies only conscious decisions.

Your subconscious brain is part of the same self as "you". So whatever you think as your "free will" is also part of that, just that part of it plays as "voice inside your brain" (conscious thoughts) and some of it does not.

It's not like the subscoscious processing is done by some other entity and imposed upon you.