Alright, you've said a lot Radarmark. I will give you this much: through radio-carbon dating, if we are to rely on this dating method alone, we can place the Dead Sea Scrolls as early as c203 BCE, but as late as 68CE. Mind you this is not even the entire Hebrew Bible. Just fragments. Before then, through secondary sources, we have 1. the name David 2. the name Ahab and his battle 3. the name Israel, and 4. the name Yahweh, without any details as to who or what Yahweh was. I'm not really sure what other fragments you are referring to, but I'm pretty sure that whatever they are they are not secondary sources, and that they have been dated through extralinguistic or paleolinguistic methods. Not radio-carbon dating. Given this, as I have explained, the extralingusitic evidence is scanty (a few names and battles, and nothing to imply monotheism), and the paleolinguistic evidence is easily refutable. So what if the language in a portion of Genesis shows archaisms. Lithuanian is a living language and shows archaisms too, and though is not identical to Sanskrit or Avestan resembles Sanskrit or Avestan.
If we are to use astronomy to date Genesis. Well, as I have explained, the creation of the 7 days is aligned with the Julian Calendar, which does not resemble the calendar's that the ancient's used, and implies that Genesis, "the oldest portion of the Hebrew Bible" may well have been written after the Julian Calendar was introduced c.46BCE, which more or less coincides with the later date of the Dead Sea Scrolls.
As for the Zarthushtrian scriptures. At least a portion of them were composed by Zarathushtra, himself, because they were written in FIRST PERSON. The earliest and fairest date I can find for these scriptures, if you find the Greco-Roman dating 6000BC to be absurd, is 1500BCE because the culture described in the Gathas agrees with Yaz Culture Complex. But, whether Zarathushtra himself existed is not the point. The point is that the Gathas, Avesta, Aryan Bible, what have you were the product of the collective psyche of the Aryan people. In other words the Aryan Bible existed, and it had a huge impact on the way the Jews, Christians, and Muslims think. I see Zarathushtra as a sort of Homer or Shakespear. The difference between these figures and Moses, for example, is that people really think Moses existed. Which I think is the most psychotic thing in the world.
Now, the pesamist or uninformed scholar, places Zarathushtra immediately before Darius, or during Darius's reign. Because they mistake Vishtaspa "the Constantine of the good faith" with Hystapes, Darius's father. But they were obviously not identical figures. But the Acheamenids like Darius were Mazdayasnas "Mazda Worshippers," and this much is clear from their inscriptions. The Acheamenids worshipped Mazda exclusively. Xerxes's inscriptions may be the clearest example of this. The Greeks c. 400 BC didn't fully understand the Zarathushtra ideology, but they were aware that Zarathushtra was the father of the magi, the son of Mazda Ahura, and that the Persians or Acheamenids were his followers.
The Hebrew Bible, on the other hand, there are no secondary references to until Josephus. The Dead Sea Scrolls c203BCE-68CE, can only be authenticated through secondary references e.g. the Tel Dan or Tele Mesha Stele OR the Cyrus Cylinder. This is the only way scholars have been able to authenticate the events described in the Hebrew Bible actually happened. And they don't have anything like that for Abraham, or Moses. As a matter of fact, scholars have even proposed that figures like Abraham and Moses were based on Hindu traditions e.g. a-brahmin "non-brahmin" and the Laws of Manu.
The Dead Sea Scrolls themselves show influence the Zarathushtrian religion. One of the scrolls is called quote and quote "Zostrianos." It describes a metaphysical comparable to a resurrection that resembles Zarathushtra's conference with Ahuramazda and the 7 Amesha Spenta's or Archangels.