To be precise, the biblical picture is that Israel was called from the start to monotheism, but was mixed from the start and tended to go after other deities. Of course, there’s an issue in defining who the earliest Jews were.
The idea that the material of the Old Testament long existed only in oral form, to be written down much later, reflects the situation in northern Europe, but not in the ancient Near East. Having a simple alphabet made it easy and profitable to keep records, and anything important was written down. Although some updating and editing continued for a long time, the basic law system of the Torah lies behind the historical narratives and pre-exilic prophets. The acceptance of the Pentateuch by the Samaritans is not compatible with its invention in the exile.
The idea of fixed patterns of development of ideas is not realistic. There’s no reason that the idea of monotheism had to develop gradually - ideas can follow all sorts of patterns (e.g., Akhenaten’s monotheism, almost certainly pre-Moses).
The only actual data that we have on the development of Hebrew theological ideas is the Old Testament. Of course, one must judge its reliability, but the approach of claiming that a particular feature must be late and rejecting anything that points to an earlier date on the grounds that it conflicts with the premise is the chronological approach employed by YEC as well as by some critical biblical scholarship.