Context for proto-Judaism might come from Simo Parpola's theories of a Neo-Assyrian origin for Jewish esoteric beliefs. While Parpola is highly respected, other Orientalists discount some of his more assertive claims as inappropriate. Nevertheless, what Parpola highlights is a style of religion - the ascetic prophet connecting with the holy spirit - which defined the middle period of Judaism. It would appear that the Age of Prophets is Jerusalem's reaction to Assyrian cultural hegemony. The common Syrian/Canaanite basis for Israelite and Assyrian religion would make it even easier to develop a "Judah brand" of the Assyrian national religious practice.
Where Parpola reaches is identifying sefirotic symbols among Assyrian religious iconography. The sacred tree motif is replete in Assyrian carvings, including diverse geometries and forms. Parpola speculates that these are sefirotic trees represented exoterically and impressionistically. Moreover, he posits a Neo-Assyrian origin for Kabbalistic systems.
Whatever the truth of that, his exegesis on Assyrian monotheistic trends through a Kabbalistic interpretation provides a fascinating insight into proto-Jewish monotheism.
Parpola very speculatively constructs a Neo-Assyrian Tree of Life, inserting Babylonian gods into his Assyrian Kabbalah. This is certainly a stretch, but in his deep understanding of Assyrian motifs, he is able to conclude the following:
(Cooper, Jerrold. Assyrian Prophecies, The Assyrian Tree, And The Mesopotamian Origins Of Jewish Monotheism, Greek Philosophy, Christian Theology, Gnosticism, And Much More)"The ease with which the gods and their numbers fitted into the diagram was almost too good to be true... I felt on the verge of a major discovery." Parpola continues: "practically all the great gods of the Assyro-Babylonian pantheon figure in" the diagram, except Assur. "This strongly suggests that this important god has to be identified with the winged disk over the Assyrian Tree... and, accordingly, is identical with the transcendent God of the Kabbala, En Sof."
There are plenty of problems with this, suffice it to say that Assur's role in the Assyrian religion is subdued in spite of being supreme. Parpola interprets this as subtlety, but that might not be the case.
If such an understanding was transposed into Jewish thought (as an evolution of polytheistic or monolatristic Judaism), then the consequent system is as follows:
1) Yahweh as the supreme divine essence, formless, unknowable, ineffable.
2) Joshua the High Priest - effectively, God. The active power of the universe. All other gods being puppets or masks, aspects of the hidden or subtle power. Perhaps overlapping with notions of Logos. In Syrian mysticism, it's Al-Khidr. The Green Man. It's shaman Pan, transposed into the character of a metalsmith or "Great Architect". The Tekton.
3) The Holy Triad - in Judaism, the Davidic King as the Adonic son of Logos/Sophia (i.e.: Pan and Gaia). Together the ruling power of Israel.
4) The gods, later: angels, patriarchs, giants, jinn, etc.
In proto-Judaism, therefore, there could have been a template for monotheism where it is not omnipotent and unitary sky father who rules creation. Rather, it is a system of aspects, divisions, gods and powers. Joshua the High Priest is the hand of the ineffable essence. In this sense, Joshua is the one true god, and template for monotheism.
The omnipotent, unitary sky father comes from Greek philosophy, the Monad.
Therefore, even before Pentateuchal Judaism, there was an expression of monotheism in Judaism. However, this one true god was not Yahweh, but rather Joshua. Seth. Adam. The Tekton. The hidden power behind the powers.
As Pentateuchal Judaism presented Yahweh as a monadic one true god, Jewish tradition and mysticism would center around an entirely different god. That's where we find Gnosticism, Christianity and so forth.