"The answer doesn't matter."

Is there a God, or some sort of ultimate ground of being?

The answer doesn't matter, unless these questions have meaningful answers:

1. Does God v. No God change what information you have access to? If there is a Mind of God, can it change your mind? How? (Only through other people, books written by people, but not directly? If so, how?)
2. How do we know who has access to the Mind of God? What do we know and how do we know it? Who can know it? Why don't they get the same answers? etc.

If the Mind of God scenario is indistinguishable from the No Mind of God scenario, then effectively we are all atheists.

Similarly if Mahamudra or teachings of Nagarjuna posit a perspectival-only view of the universe, does it matter if there is an ultimate, unified ground of being (God, ultimate reality, monad, a Cosmic Buddha, etc.) or not? If that doesn't change what you know and how you know it? Is Mahamudra an atheist or theist endeavor? It doesn't matter.

Also, if there is no God, then atheism is a word for just "the way things are." In that case, what are theists, if there is no God? They are believing in what exactly?

The only people who claim that whether God exists matters are those who gain or lose the most based on the answer. In other words, ordinary human politics. The loudest are the ones who benefit the most from maintaining the status quo. Everyone else only cares if God can help or hinder them. And if so, how do we know? Back to epistemology. What do we know and how do we know it? Everything else is irrelevant.