Toward a Leibnizian Cosmological Argument Against Theism

So here's a sketch of an argument I'm toying with that's in the same vein as several others I've discussed here recently:

Suppose you think, along with a number of proponents of the Leibnizian cosmological argument, that at least the following version of PSR is true: every object has a sufficient reason or explanation for existence, either in terms of (a) something else or (b) its own nature. Call beings of the former sort 'contingent beings', and call beings of the latter sort 'necessary beings'. Suppose further that you think (again, along with proponents of the Leibnizian cosmological argument) that contingent beings can't be fully explained merely in terms of just other contingent beings, no matter how many (and for the usual reasons offered). If you grant all of this, then it looks like you have good grounds for thinking that at least one necessary being exists that explains the existence of contingent beings.

However, I worry that the train of reasoning the theist has invited you to follow will take her where she doesn't want to go. For it seems to me that there are good grounds -- grounds at least as good as those in support of PSR -- for thinking that all concrete stuff that has an originating cause or a sustaining cause must have a material cause. But if so, then if classical theism entails that contingent beings require an originating cause or a sustaining cause without a material cause -- i.e., if classical theism entails that God creates or sustains the universe ex nihilo -- then the God of classical theism cannot provide a sufficient reason or explanation for the existence of contingent beings. Rather, it looks like matter -- or whatever matter is ultimately made of -- is the only kind thing or stuff around that can suitably occupy the role of necessary being.

In short, PSR seems to entail the falsity of classical theism. Indeed, it looks like the prospects don't look too shabby for an argument that PSR entails naturalism (or at least something close enough to naturalism for classical theists to find unpalatable).