The DSS offer us the earliest existent forms of most of the OT. Clearly not all the documents found there are scripture, and I know of no one who makes such a claim. However, they can be valuable in gaining insights.rewriter
You are woefully lacking in referring to the Dead Sea texts as a validation for versions of the Old Testament because modern Jews and Christians reject the cannon of the Essenes.
The non-canonical books most frequently found at Qumran were the Book of Jubilees and the Book of Enoch (1 Enoch). These apocalyptic texts were incredibly popular with the Essene community, surviving in multiple manuscript copies.
Other widely read non-canon writings at Qumran included:
The Book of Tobit: A deuterocanonical book found in six copies.
The Wisdom of Sirach (Ben Sira): Found in several Hebrew fragments.
Testaments of the Patriarchs: Including various copies and adaptations of the Testaments of the Twelve, such as the Testament of Levi.
Sectarian Qumran Writings: Original compositions unique to the settlement, such as the Community Rule, the War Scroll, and the Temple Scroll.
You quote Codex Sinaiticus as a source but The Codex Sinaiticus (c. 330–360 CE) is another nearly identical early manuscript. It holds the oldest surviving complete copy of the New Testament, along with roughly half of the Old Testament. Codex Sinaiticus conversely and uniquely includes books like the Epistle of Barnabas and The Shepherd of Hermas as sacred books.
A more complete listing of versions of the Hebrew Bible include:
The Septuagint (LXX): The earliest translation of the Hebrew Bible, rendered into ancient Greek in Alexandria, Egypt, between the 3rd and 2nd centuries BCE. It contains variations in text and order compared to the Masoretic Text and includes additional writings (often called the Apocrypha).
The Dead Sea Scrolls (DSS): Discovered in caves near Qumran between 1947 and 1956, these ancient scrolls date from the 3rd century BCE to the 1st century CE. They contain fragments of every book of the Hebrew Bible (except Esther) and provide vital evidence that multiple textual versions circulated concurrently in antiquity.
The Samaritan Pentateuch: An ancient Hebrew version of only the first five books of Moses (the Torah), preserved by the Samaritan community. It is written in a paleo-Hebrew script and features unique textual variants that emphasize the sanctity of Mount Gerizim.
The Targums: Aramaic translations and paraphrases of the Hebrew Bible that arose after the Babylonian exile when Aramaic became the common spoken language of Jewish communities.
The Peshitta: The standard Syriac translation of the Bible, created for the Aramaic-speaking Christian communities in the 2nd and 3rd centuries CE.
The Vulgate: A 4th-century CE Latin translation by Jerome, who translated the Old Testament directly from original Hebrew manuscripts, which became the standard text of the Catholic Church
As for citing Codex Sinaiticus and various Codices and targums, among others as you cited, they, like different modern-day translations are known potential sources and can offer insights.
Now the question as to why the Jews in the late 1st century chose the Masoretic Text as the official version of the OT is a question that could make for a very interesting discussion, especially since Christianity later also adopted it, even though from among the DSS and the Septuagint, it has the fewest undisputable connections to the NT. The DSS are clearly cited as is the Septuagint, but the Masoretic appears only in places where it reads the same as one or both of the other two. And I would note that both Jubilees and Enoch appear to be cited in the NT.
I also trust that you are aware that both Barnabas and the Shepherd were among the works considered for inclusion in the approved NT, so their inclusion merely acknowledges that the final canon of the NT was still fluid at the time of the writing of Sinaiticus.
I suspect that you and I could have several enjoyable sessions over beverages of our choice while discussing scripture.