How would you have Zoroastrianism survive until modern times at or above about 5% of the world's population, while not making Islam smaller than Zoroastrianism, or making any serious dents in the other 3 major religions?
Not sure if this is ASB or not, but an alternate history mod I made for a game entitled Europa Universalis III had part of the Sassanid family flee east along with a portion of the population (POD being a weaker Islamic offensive), taking better defensive positions there, the result being an Indo-Persian Zoroastrian state.
Have several Kingdoms in India, East Africa and Central Asia adopt it as their state Religion and have a native Zoroastiran population grow there as well. Islam didn't really penetrate those areas until the 900s-1100s.
Meanwhile, you could help the Religion by having the Zoroastrians become something like the Copts in Egypt.