I have prepared this summary of a related Thomist argument to the OP's argument for the sake of the discussion. I found it is probably more related to Buddhism (causality, process) then the one given by the OP (being itself). The Buddha teaches causation, so this seems to be native ground. Let's discuss its content. The Buddha never seemed to get beyond the idea of infinite regress. If that is not the case, then how can we explain him not getting beyond it, or where is the error in logic below?
Some terminology to start.
First or principal causality: Has its causal power inherently. Ultimately, pure actuality.
Second or instrumental causality: Derives whatever causal power it has from something else.
Essentially ordered, ordered per se: All the causes in such a series other than the first are instrumental, for their being causes at all depends essentially on the activity of that which uses them as instruments.
Examples on the natural plane: A hand using stick to push a stone.
Accidently ordered, ordered per accidens: All the causes in such a series do not essentially depend for their efficacy on the activity of earlier causes in the series.
Examples on the natural plane: A father possesses the power to generate sons independently of the activity of his own father, so that a series of fathers and sons is in that sense ordered per accidens rather than per se (though each member of such a series is also dependent in various other respects on causal series ordered per se).
Now, the first cause St Thomas argues for is “first” not in a temporal sense, but in an ontological sense, a sustaining cause of the world here and now and at any moment at which the world exists at all -- the absolutely fundamental cause, that apart from which nothing could cause (because nothing could exist) at all.
First, why does Aquinas hold that only God can possibly create out of nothing? Here’s one way to understand it. Any of us can easily actualize the potential of the oxygen in the air around us to move, simply by waving our arms. Only someone with the relevant expert knowledge could take oxygen and hydrogen and synthesize water out of them. It would take greater power still to cause the prime matter underlying oxygen, hydrogen, or water to take on the substantial form of a tree. But creation out of nothing requires more power even than that, in fact unlimited power. For it is not a case of drawing out the potentialities that are already there in a thing, but rather causing a thing to exist entirely, together with its potentialities, where nothing at all had existed before. It isn’t a case merely of modifying what already exists, but rather of causing to exist in the first place that which all mere modification presupposes.
Limited causes are limited precisely by potentialities which are not actualized. Hence a sculptor is limited by the degree of skill he has so far acquired, by the limits on his dexterity given the structure of his hands, etc. He is limited also by the potentialities of his materials – their capacity to be molded using some tools but not others, their capacity to maintain whatever shape the sculptor puts into them, and so forth. Now that which creates out of nothing is not limited by any such external factors, precisely because it is not modifying anything that already exists outside of it. But neither can it be limited by any internal potentialities analogous to the limits on a sculptor’s skill. For it is not merely causing a being of this or that sort to exist (though it is doing that too) – modifying preexisting materials would suffice to cause that – but also making it the case that any being at all exists. And only that which is not a being among others but rather unlimited being – that which is pure actuality – can do that.
The idea is perhaps best stated in Platonic terms of the sort Aquinas uses in the Fourth Way. To be a tree or to be a stone is merely to participate in “treeness” or “stoneness.” But to be at all – which is the characteristic effect of an act of creation out of nothing – is to participate in Being Itself. Now the principle of proportionate causality tells us that whatever is in an effect must be in some way in its cause. And only that which just is Being Itself can, in this case, be a cause proportionate to the effect, since the effect is not merely to be a tree or to be a stone, but to be at all.
So only God – who just is pure actuality or Being Itself rather than a being among others – can cause a thing to exist out of nothing. But why could He not work through instrumental causes in doing so? For all the preceding argument would seem to show is that Being Itself is the ultimate cause of any thing’s existing at all. That is, it suggests that any cause of a thing’s sheer existence that was less than Being Itself would, either directly or indirectly, owe its own existence to that which is Being Itself. But why couldn’t that which is Being Itself impart to other things their sheer existence through such an intermediary – through an instrumental cause which, like the effect, is merely a being among others rather than Being Itself?
Here’s one way to think about the problem with this idea. An instrumental cause causes by virtue of being used to alter what already exists, as a chisel is used by a sculptor to alter marble. But to cause the sheer existence of a thing out of nothing is not to alter what already exists. In the case of a material thing, it does not involve causing already existing matter to take on a new form (as a sculptor does), but rather causing the matter and form together to exist. Hence while it makes sense to speak of using a chisel in the act of sculpting a statue out of marble, it makes no sense to speak of using a chisel in the act of causing a statue to exist out of nothing. For before the statue was caused to exist out of nothing, there was no marble on which the chisel could be brought to bear; and after the statue is caused to exist out of nothing, there is nothing for the chisel to do, since the marble already is (by hypothesis) a statue. Now any purported instrumental cause involved in any act of creation out of nothing would be like the chisel. It would be a fifth wheel – it wouldn’t be doing anything, and thus would not be causing anything, and thus would not really be an instrumental cause (because not a cause at all). Hence the very idea of God creating out of nothing through instrumental causes falls apart on analysis.
So, while popular images of God as First Cause have Him knocking down the first domino billions of years ago, and while even Aquinas might seem to make of Him the distant terminus of a regress of simultaneous currently operating causes, nothing could be further from the truth. God’s relationship to the world is in Aquinas’s view much more intimate than that, indeed, as intimate as possible. At least where the sheer existence of things is concerned, He and He alone is directly causing them at every instant. He is, as the Muslims say, “closer than the vein in your neck.”
Such a series can be simultaneous (present) or sequential (through time). It is ultimately their instrumental character which makes every member of a per se ordered causal series other than the first depend necessarily on the first. And the Thomist does hold that the world must ultimately be sustained at every instant by a purely actual uncaused cause, not merely generated at some point in the past.
Now, one may state, “every member of the series is genuinely the cause of the one that follows it.” Now if this assumption were correct, then it would indeed be odd for Aquinas to hold that a series of causes per accidens might be infinite while a series of causes per se could not be. For it is precisely because they have their causal power independently of any earlier members of the series that Aquinas argues that the activity of the members of a per accidens series need not be traced to a first cause. So, if he thought that the members of a series of per se causes also had independent causal power, then his reason for tracing that sort of series to a first member would be undermined. But of course, that is not what Aquinas thinks. He thinks that they do not have such independent causal power, and so it is not at all odd, arbitrary, or unjustified for him to say that a series ordered per se needs to trace its activity to a first uncaused cause. Many miss this because they think that the Thomistic argument rests on an appeal to simultaneity, and they don’t see how simultaneity requires an uncaused cause. But as I have said, the argument doesn’t rest on simultaneity as such. It rests on the instrumentality of the members of a causal series ordered per se, and instrumentality does require an uncaused cause.
[As a side note, this does not mean that there is no sense in which the members of a causal series ordered per se are genuine causes; Aquinas is not an occasionalist. But how his account avoids occasionalism is a separate issue, and does not affect the soundness of Thomistic cosmological arguments as such.]
It wouldn’t change things in the least if we granted for the sake of argument that a series of causes ordered per se might loop around back on itself in a circle, or even that it might extend forward and backward infinitely. For the point is that as long as the members of such a circular or infinite chain of causes have no independent causal power of their own, there will have to be something outside the series which imparts to them their causal efficacy.
(As the Thomist A. D. Sertillanges once put it, a paint brush can’t move itself even if it has a very long handle. And it still couldn’t move itself even if it had an infinitely long handle.) Moreover, if that which imparts causal power to the members of the circular or infinitely long series itself had no independent causal power, then it too would of necessity also require a principal cause of its own, relative to which it is an instrument. This explanatory regress cannot possibly terminate in anything other than something which has absolutely independent causal power, which can cause or “actualize” without itself having to be actualized in any way, and only what is purely actual can fit the bill.
That is the way in which it is “first” – first in the sense of being metaphysically ultimate or fundamental, and not (necessarily) in the sense of standing at the head of some (temporal or even non-temporal) queue. That is also why, contrary to what many atheists suppose, it makes no sense to ask why fundamental physical particles or the like might not be the first cause. Particles and other “naturalistic” candidates for the ground floor level of reality are all compounds of act and potency, form and matter, essence and existence [i.e., inconstant]; accordingly, they are in need of actualization and are therefore necessarily less than the “pure act” or Subsistent Being Itself which alone could, even in principle, be that which causes without in any way being caused (or, as I would prefer to say, which actualizes potency without itself being actualized).
[Culled from Edward Feser's article, "Edwards on Infinite Causal Series" and "A first without a second"]