Hello Rabbi,
I’m familiar with the Rabbi’s position regarding infinite regress and so on in the question of the eternity of the divine.
I was thinking to myself a strange thought: does God Himself wonder how He was created? Or more precisely, is God capable of thinking of/about Himself?
I warned in advance that it’s strange. Sometimes the issue occupies me.

Next time I see Him, I’ll ask and let you know.

Even if He isn’t complex, something like that still requires an explanation for why it exists. So again, why do we choose to draw the line at God and not at the world? It feels to me like we use God as a joker card, and with Him we stop asking questions.

He is exactly the joker card. The ultimate regress-stopper. This argument proves that a joker exists. Excellent metaphor.
The problem is that you insist on continuing to ask even where there is no question. And what seems strangest to me is that you accuse believers of stopping their inquiry when they’ve found an answer. “One does not answer back to the joker.”

“Beyond that, a complex thing that was created needs a composer. But if there is something complex that has always existed, it does not need a composer” — doesn’t that contradict the principle of sufficient reason (as you wrote in Notebook 2)?

No, for two reasons(?!): 1. He is not complex, so there is no space of possibilities here that would make us wonder why this one in particular exists. 2. Without the joker, you end up in an infinite regress. Hence it must be an entity that requires no cause, and therefore no reason either.