It might be less arbitrary to speak of a range of divine thoughts as being fundamental. But metaphysical idealism in general seems arbitrary from a materialistic, science-centered perspective, because of its anthropocentrism.
Math is at least arguably fundamental to rationality and perhaps even to science, so that seems to be the Illuminist's starting point. I don't know enough about original Pythagoreanism. I believe they were mysticists about math, practicing numerology and divination. He seems to have taught reincarnation, so maybe he traveled far eastward, beyond Egypt, perhaps to India? He's a mysterious figure.
The cosmicist point I make about any kind of metaphysical idealism, theism, or mysticism is that this kind of revelation (mind as fundamental) should be horrifying. The force of it should send us into the wilderness like John the Baptist. The rest is hot air. Love of God is for suckers who are being conned. Fear of God is a sign at least that we've understood the gravity of a theistic situation.