Your a priori argument leads to the conclusion that there is a philosophical God.
After reaching that conclusion, you reject Russell’s “teapot” argument by saying that since I already know of the existence of a philosophical God, it now becomes much more plausible to accept testimony of an encounter with Him. We just need sufficiently reliable testimony.
From there you narrow the options down to the monotheistic religions.
As one of the significant differences between the revelation story at Mount Sinai and other myths, you write the following in your book The First Existent:
“It should also be added that if this were indeed an artificial implantation of a myth about revelation, we would expect the content of the revelation to resemble what was accepted in that period—that is, pagan and polytheistic—and not a monotheistic myth speaking of an abstract God who makes strange demands of us…
The God at Mount Sinai remains abstract and is not described in anthropomorphic terms like in the other mythologies. It is not plausible that a people would invent a myth so different from what was accepted in its cultural environment.”
This argument is based on the claim that Judaism in its original form really does speak of one abstract God, and the argument rules out other religions that do not fit those criteria.
So here is my question:
If I were to prove to you, or convince you, that Judaism in its original form did not speak at all about an abstract God, would that not damage your argument?
After all, your argument is based on a combination of an a priori philosophical conclusion that an abstract God exists on the one hand, and on the other hand a tradition about that same God revealing Himself to human beings.
But if you were to conclude that the second direction is not talking about that God at all,
how could that not hurt the argument?

Now I understand what you mean. That is a completely marginal argument in the overall structure. True, it would be weakened if your description is correct. Even so, it is clear that there was a stage at which we adopted an abstract God, while the surrounding world had not yet done so (before the Christian era). So in principle it still stands.

But even if it still stands, in that case the revelation at Sinai is not speaking about the same God.
Also, there is serious doubt about your claim that we adopted an abstract God before the Christian era.
It may be true that some among us adopted that view, but it was definitely not a principle of faith accepted by all of the Jewish people.
Rabbi Moses Taku, for example, went to war, as it were, on behalf of all those with pure faith, and brought many proofs from all the sources of Judaism that the Creator has a form, arguing that anyone who denies this denies the Written Torah and the Oral Torah. God-fearing and upright Jews, who spent all their days immersed in Torah and Talmud and drew all their knowledge from Jewish sources, apparently did not know that all the corporeal verses in the Torah were merely metaphors…
Likewise, regarding Maimonides’ statement that belief in a form or corporeality is heresy, the Raavad comments:
“Why did he call such a person a heretic? Many greater and better than he went with this belief, based on what they saw in the verses, and even more so on what they saw in the words of the aggadah, which confuse the mind.”
This implies that belief in an abstract God was not known throughout all Israel.
So how, then, can one accept the revelation at Sinai, if I understand that it is not the same God at all as the one I arrived at through philosophical arguments?

You’re just being stubborn. First of all, the discussion is hypothetical. Everything is on the assumption that it will indeed be proven that this was the view. Second, even if you proved that, conceptions of God develop, so that is not necessarily a contradiction between the later conception and the earlier one. It takes time to digest things. And finally, what the Raavad wrote proves nothing at all. So there were a few people who held corporeal views. So what? And Rabbi Moses Taku is really a crushing argument. There was also Rabbi Musa al-Baghdadi, whose manuscript was found in caves in Kazakhstan, who wrote that there is no God at all. Well, I think we’ve exhausted this.