Sadducees had many sins and vices, but they were strict monotheists. They could be accused of many things, but not of leading Israel towards religious idols (Certainly, money is an idol, but this goes across religions and times, right? we're not taking about that form of idolatry in this moment).
So the question remains: why were the priests so strictly monotheist by the time of Jesus?
I don't think the question remains. These priests, like their forebearers, didn't really believe in anything other than lining their pockets. Whatever they may have professed is largely irrelevant - it's just whatever they thought their
marks wanted to hear.
You mention that polytheism had fell out of fashion in the Middle East. I agree.... and interestingly, not in the West (Greece, Rome).
So, why was that? You mention that people have turned to a more abstract idea of God or gods. Why was that?
The two groups of gods had different literary purposes.
The elder gods (titans, as it were) were nature-centric because their original purpose was to explain natural phenomena. Why does it rain in Spring, and where did the world come from?
The latter pantheon was abstract because its purpose was word-smithing. There was a crisis of language as the Middle East shifted from using runic language (cuneiform) to phonetic language. New words were needed for abstract ideas. Myths were created around gods who epitomized ideas, and then the names of those gods were added to the vernacular to represent those ideas in conversation. What is wisdom, or desire?
Ahura Mazda was the first Universal (not national) God to be conceived in the abstract. He had no shape. Fire was used as a symbol of Ahura Mazda, precisely because "He" had no shape. We could feel its vivifying effects (light and heat), but not grasp it. Later on, Jesus and the apostles also used the metaphor of the fire to represent the Holy Spirit from God.
So your contention here is that monotheism sprung from the seed of Zoroastrianism, which was then propagated across the Middle East, presumably with the spread of the Persian empire? That probably happened in some degree, but I find it hard to think that this is a single-source phenomena.
I also find an observable evolution of religion from polytheism to monotheism. The transition seems to come through wisdom traditions. I called them gnostic earlier, but they're more like proto-gnostic. Essentially, the latter gods came to be thought of as
emanations or
aspects of a single higher God.
And there are instances of this thinking that pre-date the Persian Empire. For instance, we find that Ninevah (in Assyria) had a period of monotheism in the 8th century BC. That's 250 years before Cyrus would have theoretically brought that thinking to the region.
Even in the Bible, we have Solomon (c. 1000 BC) anthropomorphizing Wisdom mere verses removed from declaring a single Lord. For Solomon, Fear and Wisdom were emanations of the one God - a position in-between gross polytheism and strict monotheism.
Anyhow, this got long. If you read all this, thank you for your indulgence.