I've finished with Ed Feser's treatment of Aquinas' First way in Aquinas, but he did not satisfactorily (or intelligibly for me) address my question: How do we know the unmoved mover for any given change must be single and lack potential?

He gives what looks to me as "a hand-waving argument", i.e. skipping a step assuming the reader sees what he's getting at:

Consider how [a hand moving a stick to move a rock to move a leaf] would have to continue beyond the point at which we left it, with the hand's potentiality for motion actualized by the arm, [...] the muscles' potentiality [...] by the firing of certain motor neurons, and so on [...] [T]his depends [on] the nervous system, which depends on [molecules, electromagnetism, gravitation, the weak and strong nuclear forces, etc]. To account for the reduction of potency to act in the case of the operations or activities of the hand [etc] we are led ultimately to appeal to the reduction of potency to act vis-a-vis the existence or being of ever deeper and more general features of reality; for "it is evident that anything whatever operates so far as it is a being" (QDA 19). But the only way to stop this regress and arrive at a first member of the series is with something whose very existence, and not merely its operations or activities, need not be actualized by anything else. This would just be something which, since it simply exists without being made to exist by anything, or is actual without being actualized, is pure act, with no admixture of potentiality whatsoever. For suppose it had some potency relevant to its existence (its existence being what is relevant to its status as the end of the regress as we have continued it). Then either some other thing actualizes that potency, in which case we haven't really stopped the regress after all [...] or some already actual part of it actualizes the potency, in which case that already actual part would itself be both pure act and, properly speaking, the true first mover. Now, having no potency to actualize, such a being could not possibly change or move. Thus [...] not only unmoved, but unmovable.

The part I've bolded is where I get lost: What does he mean by 'deeper and more general'? I suppose by 'deeper' he means 'smaller', such as the existence of quarks. By 'general' I suppose he means how the Coulomb force appears the same everywhere in space. Yet why must this lead to God, rather than stop at the material level? Why can't the quark be a necessarily-existing being not deriving its existence from anything else? Why can't the constancy of atomic forces be a brute fact just as "God is love" (or God's nature is love) is a brute fact? I do not see why this per se series of movers must terminate in something beyond our physical universe, and I don't think he clearly explains this point: He merely asserts it.

Moreover, I don't see that his argument that the unmoved mover must lack any potential is true, and I think I even have an argument proving it false: A metal beam could have wet paint on its surface whereas it has the potential to change temperature. Whether it's at 20 C or 25 C is irrelevant to its causal ability to smear paint on something else that comes into contact with it. The metal beam with wet paint is not the unmoved mover of a ball getting paint on it because of course it depends on its atomic structure remaining intact, but since the only difference between an instrumental mover (one that is actualized by another) and the ultimate unmoved mover is whether a potential with respect to the observed change is actualized by another, it appears possible that the unmoved mover of that series could have potential regarding some other interaction.

So we can conceive of an ultimate mover that has some potential with respect to something else, other than what it happens to be moving at that time. Returning to Feser's hand-flexing example, it appears to me plausible that the electromagnetic and atomic forces, together with the quarks, could be a collection of unmoved movers, even while the electromagnetic force has the potential to be overwhelmed by the gravitational force (suppose a massive object like a black hole is moving towards the flexing hand), i.e. its effects cease to be observed because the quarks instead move due to gravitational influence. (I need to check my physics, actually, because I think the strong force is billions of times stronger than the gravitational force -- but I think we can conceive of a thought experiment in any case where the per se series could be thwarted by the introduction of another entity, and then it would seem the unmoved mover has some potential with respect to that previously-absent entity, such as injecting another quark into a molecule changing its atomic makeup, changing the physical forces acting thereon.)

Would you please specify my error and explain how I'm wrong? Why must a per se series terminate with a cause that lacks any potential, rather than merely lacking potential relevant to the observed cause? Would you please elucidate Feser's argument for me if he is correct?