There was surely a network of bridges between polytheism and monotheism, namely the henotheistic or monolatristic tendencies of polytheists to worship one god as supreme. Zeus, for example, was much more powerful than the other Greek gods. So Jewish monotheism would have had multiple sources.

The Thomistic point about the necessary being's lack of (mere) existence is like Heidegger's distinction between Being and beings. The necessary being wouldn't exist as a mere contingent thing along with everything else. God could never be present as anything particular, which makes God equivalent to nothing, to no single, conceivable thing. Would God, then, exist? It depends on whether that word is reserved for knowable particulars. God might subsist or something like that.

My interpretation of Judaism is certainly not standard. I hardly speak for all Jews. But I think my take is presupposed by Jews' relative distaste for theology.