I've said this before on Lemmy, but free will is observer-dependent, because when people say "X has free will", what they really mean is "I can't predict the behavior of X". QM doesn't really change that, it only really affects the theoretical limit of invoking some sort of advanced science or superintelligence. Is it possible that we could build a computer so advanced that it can predict all of humanity's actions as easily as we can predict the trajectory of a thrown rock with physics? To it, humans wouldn't have free will. To any individual human, other humans would still appear to have free will though. QM might provide an upper limit on how much one can predict the universe, or maybe a superintelligence could pierce the veil and determine more than we currently think is possible. Also, mechanistic interpretations aren't entirely ruled out for QM, just certain formulations of them. They're not in vogue, but not proven wrong:
https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory#Occam's-razor_criticism
Our main criticism of this view is on the grounds of simplicity – if one desires to hold the view that ψ is a real field, then the associated particle is superfluous, since, as we have endeavored to illustrate, the pure wave theory is itself satisfactory.
Why doesn't the weather have free will? It seems like a silly question, but actually gets right to the point.
What exactly is the "experience of free will"? To me, that sounds a lot like "I can't predict my own behavior", which in turn is exactly "I can't predict the behavior of X" as above, where X is oneself.
To me, this sounds like it agrees exactly with free will meaning “I can’t predict the behavior of X”. Why is it necessary for free will? Because what you actually mean by free will is "unpredictability".