I believe your reading is correct; it is also the reading of Mary Boyce.

I tend to subscribe to the idea that Middle Persian literature preserves only a portion of what was a wider spectrum of cosmological views within some constraints of orthodoxy (think of something like the six orthodox/vedic schools of Hinduism), with differing interpretations of the meaning of the Yasna.

Something that should not be overlooked is that the essential statement of Gathic dualism in Y 30 presents the two primal beings not as Ahura Mazda vs Angra Mainyu, but as Spenta Mainyu vs Angra Mainyu. Mainyu is conventionally translated "spirit" as you know, but is used interchangably with manah and should also translate as 'mentality' or something similar; the cosmogony presents a primordial choice between good and bad thought which humans still face to this day. In this cosmic order Ahura Mazda seems to be something like the highest manifestation of good, proper, holy thought, viz., wisdom.

An alternative reading which to my understanding was the one of Martin Haug and which found some traction among Parsis, is that Ahura Mazda is instead more primordial than the mainyu, and that the Spenta Mainyu was a mere "emanation", thus making Ahura Mazda eternally superior. Haug infamously posited that Zarathustra's original teaching treated the yazata/amesha as mere "philosophical abstractions" and that treating them as divinitkes was a later "corruption". This reading is downright silly to me; it's fairly obvious that there was no hard line between abstract concepts and divinities in the Indo-Iranian cosmic order.

Regarding Zurwanism, much of our understanding of it comes from Christian polemic which found Zurwa, a flawed creator, an easier (and perhaps safer) target for attacks than Ohrmazd. It is therefore almost impossjble to know to what extent a Zurwanite sect existed and what they believed.

So, essentially, I don't think there's any real basis for reading "monotheism" into early Zoroastrianism.