Thank you for clarifying a bit further as you've now identified a hypothesis (cosmological argument), albeit vague, relevant to our discussion of "god" as a possible entity for your agnosticism.
I say vague because you don't identify whether this "primordial cause" is a supernatural god in the deistic sense, or just a pantheistic metaphor for whatever natural explanation science can/may provide (in which agnosticism or atheism essentially become meaningless).
This could just as well be describing the Big Bang - the start of both space and time - where asking "what happened before it?" becomes meaningless when considering a situation without time.
However, you continue on...
... by making a big 'what if', seemingly for the sake of ambiguity.
If it is the claim of a supernatural (outside the realm of science) "primordial cause" that you were referring to, then that opens it up to both scientific and logical scrutiny in which I believe that "pure agnosticism" (the lack of assigning any probability) becomes obsolete.
If I say that: from my human perspective, everything must have a cause, and as such, I have to admit that the universe itself has one, I didn't say anything yet about the nature of such cause.
First, I am opposing to the personalized concept of God, as it appears in (institutionalized) religions, a philosophical God.
Second, agnosticism means staying there. You can't say anything more. You can't say if this cause you're capable of imagining has "existence", even, or any attribute at all, since by definition you put outside the realm of your possible knowledge.
Yet, I am saying that *IF* there is a way to get out of agnosticism, to find something you can conceptualize/imagine, or be able to conceptualize/imagine further, about God, including to negate it, then, while there are many ways to
try to do that, still I am proposing that this concept of God may be a better starting point than the God as presented in (institutionalized) religions.
Note: I'm not sure it can be identified as Big Bang. As far as I know (correct me if it's not the case), Big Bang assumes there was
something, call it energy and not matter, still something open to science (in principle, even though not yet practical) which started to expand.
If so, I'm not sure that the concept of primordial cause is exactly that... It's "what causes the apparition of something out of nothing". That could mean the cause of that energy itself. I don't know. (I don't know enough about the Big Bang theory actually).
Second note: yes, "what happened before" is not meaningful within the accepted limitation of the human intellect as:
1 - conceptualizing everything
2 - in (seemingly linear) time,
3 - obeying bivalent logic.
But then again, is that all we can do? I'd start by saying yes (and that keeps me in agnosticism), and am open to ideas as to "then again, maybe not". Also, of course, if you apply Occam's razor strictly, you can get out by considering invalid any statement whatsoever (including the vaguest "there could be something else outside the realm of science as we know it"), thus getting out of agnosticism (on God) towards atheism.