The terms religion and philosophy were invented by Europeans to organise a Euro-centric worldview. Consequently, philosophy "was invented" by the Greeks and any decent thinking before the Greeks (500BC) must therefore automatically be doomed as religion. Otherwise the idea that Europeans invented "thinking" would be all wrong.
This is why both Brahmanism, Zoroastrianism, Buddhism (especially Chan and Zen) and Daoism have all incorrectly been termed "religions" when they really all are philosophical systems. Zarathushtra's concept of Ahura Mazda has little or nothing to do with the father-figure gods of the four desert religions (Judaism, Islam, Christianity and Bahai). That's my vote!
But why not cover both angles? I suggest we refer to Zoroastrianism/Mazdayasna as a philosophy which has become a religious practice. Much like Buddhism!