As far as I know the IE peoples separated into european and indo-iranian branches, and my understanding via Armstrong is that it was because the eastern peoples ‘left’ (created a diaspora) because they could not compete against the west but they could compete south and east.
As far as I know (from the record) The Indo-Iranian religion was polytheistic.
As far as I know, zoroaster lived closer to 1000bc (earlier), even if the religion was formalized (like most were) centuries later.
As far as I know zoroaster and his work were addressed to the iranian branch, and to no small degree, to intentionally cause conflcit between the iranian and indian branches.
As far as I know the indian version of religion is the closest to our EASTERN ancestral religion.
But again the western ancestral and the eastern ancestral differed greatly in that in the western, men traded with and could often defeat the gods, where in the eastern version, men were dependent upon or submissive to the gods.
The difference is that IE’s disappeared through interbreeding with the peoples of india, the persians (iranians) were boxed in between india and the semites, Although they seem to have preserved most of their genetic independence until the arab conquest. And the west were isolated by the urals, mediterranean, black sea and alps, as were india and the chinese.