[wzdd]

> until we get to the part where it's clear there is no basis for the moral duty absent a higher, non-human, source.

Casual sneaking in of "non-human" here, when a powerful human would fit just as well.

Your issue is that there often isn't a basis for anything at all except "seems to work". That's why we have the joke about the 5-year-old who keeps asking "why", exasperating its parents.

That we don't have a complete rationally-grounded framework doesn't imply the existence of God or even that it's good to act as if such a being exists. Insisting otherwise is basically a god-of-the-gaps argument.