The problems arise when you start to think what influences the soul's decisions. Does it make decisions solely based on its state? Then you have determinism. Not only state? Than what is this not-state that creates the free will? Adding some dice throwing into this mix does not sound like free will either.

That applies if you think of a soul as ‘just another mind’, but my understanding is that souls are usually seen as different. What I find problematic with them is that they can be defined to be whatever you need to get out of a logical fix eg “souls use state but are non-deterministic”.

Ultimately you’re asking what defines a soul, which is unknown as far as I can tell.

Ultimately I am asking what defines free will. The attempts at definitions that involve soul seem either circular ("soul is that part of human that has free will") or an unnecessary step, i.e. the considerations about soul might as well apply to original human, without the attempt to define a separate soul.

An argument could be that the soul is not constrained by the law of phisics or even logic and ultimately unknowable.

But then the link between the soul and the phisical world should be detectable due to violations of the laws of phisics.

Unless we believe that the whole universe is unlogical and does not follow the law of phisics. We might as well end this, and any other, debate then.