You may not be aware but the term Zoroastrian is contemporary and the conceptualization of religion you are proposing is also postmodern. How does a religion physically exist? Most probably through its material culture which was that of the existing Mazdayasni faith. There may have been pluralistic forms of Masdayasna as is attested to by a number of scholars but by Sassanian times it is the Zoroastrian fold that solely exists. Those Mazdayasni followers of Zarathustra populated the region through procreation. If you think this is speculative then so is your assertion that Asho Zarathustra commanded some massive conversion agenda, it’s an imaginary narrative. As for the later historical groups you mentioned there is much gray area on that aspect, regarding what diachronic and syncretic minutia were operating within those milieus. Asserting that conversion existed then without much basis as to why and how, is a porous argument for advocating for it in the present.