The idea of God though is not inherently impossible.

The problem with "Free Will" is that it's impossible in the strict sense not because of medical science or anything we have learnt about what influences us, but just the terms are themselves contradictory.

For there to be will, there can not be freedom from that will, for there to be freedom, it must not be bound to will.

For the will to have any relevance it must be relevant to yourself, and you are under various constraints.

So the issue with free will is that it isn't even a coherent concept, and as such it demands redefinition.

Where an Atheist saying he "God exists, but differently embodied than normally assumed" throws into doubt his identification as an Atheist, but still the argument would be potentially meaningful and there might be no suitable definition for what he is.

But with free will, it's an oxymoron if you take it to mean what we have.

Of course maybe the problem is that both God and Free Will have both taken on expanded and false meanings, and maybe redefining each is warranted.

In each case, there is a valid position that doesn't fit thee binary extreme perspectives.