Fiat currency isn't just a matter of belief or stories. If you interact with the US financial system in any significant way then you're likely to end up owing some income taxes, and those can only be paid in US dollars. If you don't pay then eventually law enforcement officers will seize your assets (real estate, cattle, gold, cryptocurrency, whatever) by force and auction them off to settle your tax debt. And if you try to stop them then they'll shoot you. Overall this is a good and stable system. The cryptocurrency clowns never seem to address the sovereign taxation issue.
While that is true, the overarching point is that it fundamentally still is, just a story. You're just describing real world consequences, precisely because humans/societies believe in that story and enforce it.
By that logic, most everything other than a club to the head is "just stories". Hume points out in a Treatise on Human Nature that most of how we interact with the world is through a coherent hallucination provided by sense organs that tell very narrow stories, but when stitched together form a cohesive whole. You're almost doing the equivalent of saying that gravity is "just a theory".
Society is a story, not a chemical reaction. People like the story of money because the stories that had gold coins or barter sucked a lot worse.
So what you're saying is that you can pay your taxes in dollars, real estate, cattle, gold, cryptocurrency or whatever.
> Fiat currency isn't just a matter of belief or stories
... Then how do we, as a society, determine how much a dollar is worth?! We do use force to enforce the stories we tell about fiat. But 'believe this story about how much a dollar will buy you and how much you owe, or else we will send thugs to your house' isn't disproving the point at all.