I agree that religion evolved roughly as you suggest, but I don't think that shows that religion preceded the existential questioning. Animists may have projected their subjectivity onto nature automatically in a naïve, childlike way, but eventually they became alienated from the wilderness as they were dragooned into farming or slaving away for sedentary societies. That alienation, fostered by civilization, would have exacerbated the doubts that arose first as people started to see themselves as being above the other animals.

When you say, then, that "Life here on Earth seemed cruel, capricious, and difficult, but there was meaning behind it all. Death wasn’t the end," you seem to be suggesting that religion was the answer to the existential fears of unfairness, suffering, and death, fears that surely preceded religion.

But whether you're implying that or not, the existential doubts weren't likely created by religion. So I don't see why they should disappear along with religion. This is why Nietzsche said the death of God is traumatic, because the existential problem remains and we've lost our ace in the hole.