I believe you are absolutely correct!!!
Furthermore, I believe that it is the Ahura part which is the original pagan Indo-European part of Zarathushtra's concept. This would make perfect sense. The Mazda part is then what Zarathushtra and/or his contemporary culture added (without removing Ahura, so thereby reforming rather changing the religion), which also explains why it is precisely the Mazda part which is UNIQUE to Zoroastrianism.
There is no concept of Mazda in Indian or European mythologies or philosophy, for example. The Indian concept of the Brahman is the same as Ahura, without the Mazda.
Arthur has brought this up before and so has Parviz. It is the Mazda part of Ahura Mazda which makes Ahuramazdaism different from any other Pantheism or Panentheism.
It doesn't even fit with any of the two labels of Pantheism or Panentheism to be honest about it. The concept is that unique.
So let's say that we have strong reasons to believe that Ahurayazda predates Zarathushtra and his generation but that Mazdayasna is the paradigm we live within afterwards. That much we should know for certain.
There is no evidence in the pantheon of Indo-Iranian deities, that a god called "Mazda" was worshipped before Zarathushtra. Professor Thieme who was one of the finest scholars of ancient Indo-Iranian texts and inscriptions, made that categorical statement in his talk at the First Gatha Colloquium held in London in 1993.
Many scholars have speculated that mazdayasna was a prexisting belief system that Z reformed. But there is no evidence that I am aware of to back up that speculation.
Wishing us the best,
Dina G. McIntyre