I suggested it was a rhetorical or semantic trick, rather than assumed it, because it seems to me a stark contradiction. Indeed, there are many religious doctrines that are quite ancient; for me, this may or may not count in their favor. Tradition itself is not enough to convince me of the plausibility of a doctrine. You should understand this with respect to the doctrines of other religions. You might see the trinity as an obvious truth, a doctrine well supported for centuries, but don't expect me to assume that it is at all plausible. Nor should you get offended when I suggest it is not. This is a matter of taking someone's views at face value, and being willing to have your views challenged.
I hardly persisted in arguing against a God no one believes in here, as a) I had only just asked what people thought the nature of God was and had certainly not begun assuming one version was true or mainstream, and b) I can hardly assume what sets of religious beliefs each individual holds, as they differ so heavily, and so the burden is on you to explain your God rather than blame me for misunderstanding your version of it.
Do you find it arrogant that you can label other religions incoherent? I suspect not. I suspect there are many serious doctrines and dogmas which you reject offhand, and I would not accuse you of arrogance for this. Moreover, I think my claim is at least prima facie plausible. If you can't even see how someone could think God is complex, then you may need to do extra work to put yourself in the other's shoes and see things objectively.
Will I be willing to accept that my arguments fail if they do? Of course. Again, I just barely started entertaining this idea, so accusing me of 'persisting in arguing' is just absurd. And your bluntness does come across as offensive - I cannot honestly say otherwise. If you can't address a serious inquirer who studies philosophy of religion, then I'm not sure what you're doing on this forum, and I will interpret your departure as a concession. Do what you will. I am confident we can be constructive if you avoid the unnecessarily critical language.
I am willing to admit when an argument is wrong, though I would say this argument only proves or disproves one conception of deism, not theism. If you're able to logically show that God is simple, that is all we will have gotten, not a further proof that if 'God is simple, then God exists'. You would still have to explain why this simple God exists. Intellectual integrity is required on both our parts.
NOW, to the argument itself
1) You've suggested that your God is the first cause. I understand this argument, contingency and necessity, and so on, a version of the ontological argument. To me, this defines your God as a 'first mover' or 'first cause.' This God is then the 'prime' thing, the 'first' thing, the 'base' thing. But I would not say that being the 'base' fact makes it the 'basic' fact or being 'first' makes it 'simple'.
2) I would contend that change is an illusion. There is one state, one slice of timespace, and then another. Change is an illusion. A heap of sand has 99 particles, then 100, then 101. There is no actual change.
Assuming that potentiality is some sort of actually real set of possibilities, one could call a choice to bring about a change the reduction of potentiality to actuality. I, myself, am a determinist, and don't think there is a field of potentiality. Hence bringing about a change would not be reductive at all, it would map from 1 possibility to 1 actuality.
By your rules of logic, the first thing cannot be potentiality, because all potentiality must have been reduced by something prior to becoming actuality. To do so, you say that actuality is necessary for causality (why not potentiality), and then that nothing can be uncaused. If the way you logically assumed that the first cause is actuality is by positing a rule whereby nothing is uncaused, you've violated your own logic by claiming that the first cause is just uncaused actuality. I see no way around this, but I will press on.
So, if I assume that there is a first cause, and that it is actual, how does it become simple? You've argued that it can't be composed of anything, because composition implies potentiality to change. But does it? Do the rules of quantum physics change? Does entropy change? Matter and energy are converted and rearranged, of course, but does this even constitute real change? I would maintain that change is an illusion. I can also think of features and properties of our universe that seem unchanging. So I dispute the premise that composition implies potentiality to change.
You've taken the idea that composition implies potentiality with the other premise (which I also have not accepted) that the first cause is actuality, to say that the first cause is not composed of anything, and is thus simple.
What makes you think that the most simple thing is the pure act of existence? Isn't non-existence more simple than existence? Isn't it more basic than existence?
Now this could almost suggest a reductio ad absurdum to me. Your proof, if valid (I cannot say this) would suggest that the most powerful, wise thing is the most simple. Then this simple thing is somehow able to be attributed many descriptors, abilities, intentions, etc. So it has many properties. It is more complex than something that lacks those properties. You are claiming that these properties are just different versions of the same facet, yet they are clearly distinct properties. I would argue that this conclusion is a contradiction. List any attribute of your God. If it is more than nothing, than it is not the simplest it can be. Thus by rejecting the conclusion, I would have to deny a premise. I would get off at the first stop, rejecting your idea that there is such a thing as causation or change.
I can see the same sort of issues, to my understanding, in your thesis. God is everything, and hence 'knows' everything. Yet these things change, hence God changes. Hence God is not immutable.