Question

Your justified position is well known: that the Hebrew Bible (Tanakh) has no normative educational value (everyone learns from it only what they already knew beforehand), and so apparently it is redundant.
What do you think of the following explanation:
In the original plan, there was supposed to be a separation between the Written Torah and the Oral Torah. The Written Torah as an ethos of a sacred text that the entire people are bound to and can see with their own eyes, and that was dictated to Moses by the Almighty Himself—even though in practice it is impossible to learn anything from it, and the Sages can knead it as they wish in order to bring supporting proof. The Oral Torah, which was never supposed to be written down, in order to preserve dynamism and adapt the will of God to the ever-renewing present.
In other words, you need a sacred written text (sitting inside sacred Torah scrolls inside a sacred ark) that has no real educational value, only in order to create a Judaism with unified Jewish law, so that it will not scatter into a thousand fragments.