My friend, you know me well enough by now, I trust, to know that I am not advocating Torah m'Sinai, just pointing to the tradition. In that regard, although the entire Tanakh is considered holy, the Torah is considered the most sacred,followed by Neviim followed by Ketuvim because the tradition sees the Torah as words by G-d, Neviim as words from G-d, and Ketuvim as words about G-d.
I would be leery in positing an opinion about how what came to be scripture was seen at the end of the 1st Temple period. There are too many unknowns, to my way of thinking, as to in what form the components existed at that time and there is, of course, the nature of Israelite religion at that time which was focused on the Temple cult. There are scholarly disagreements as to when the components were written and, for example, in regard to what are referred to as J and E, whether J and E were, prior to the final editing of the Torah, melded together as a single document JE. As you may know Richard Friedman compiled, in The Hidden Book in the Bible, his reconstruction of J. As regard to E, there is some disagreement as to whether E ever existed as a total separate entity or was just separate kinds of units.
Peter