“Classical theism” is supposed to have given us a static, immobile God.

On the contrary: One of Athanasius’ central complaints against the Arians is that they denied the inherent fruitfulness, generative power, and creativity of God.  If the Son is not eternal, “proper” to the essenceof the Father, then the Father is not inherently, eternally productive.  The fact that He has an eternal Son shows that he has a “generative nature” ( gennetikes phuseos ); only such an eternally fruitful God could have created at all.