At core, Zoroastrianism was dualistic.
I think we have to consider the Ancient Middle East as a kind of divine supermarket with all sorts of religious ideas colliding and influencing each other.
Simple version: A bunch of shepherd tribes sharing many of the stories and beliefs of the peoples around them fall under the influence of the Egyptians and acquire lots and lots of rules, an organised priesthood and military prowess - though, since there aren’t a lot of them, they’re always going to be prey to Great Power politics.
Eventually, the Zoroastrians convince them that you don’t just fade away after you die and the Divine doesn’t lose interest just because geography means you can’t follow all the rules.
What they don’t pick up, at a theological level, is the idea of that there could be a ‘God of Evil’ almost as powerful as the ‘God of Good’, ie God.
That was their legacy to Christianity.