In my view, you are pushing too far the Theory of Subjective Value (ToSV). Under the ToSV you can explain the value of a lucky rabbit foot, which many people would deem silly (including myself), but that doesn´t mean that the valuator subject is not fullfiling a need (luckiness) based on the percieved properties of the rabbit foot. No matter how silly we think that need or those perceptions are.

The fact that perception can be more or less rational does not imply that perceptions are formed in the vacuum, perceptions are based on the properties of the object that is being valuated.

Regarding money, the perceptions of the good´s properties from many different valuators must converge, otherwise the good won´t succeed as medium of exchange (MoE).

Fiat won because, like it or not, was deemed convenient as MoE by he market (otherwise the market would have immediatly rejected it as it has done many times throughout history)

The belief that something can be successful as MoE just because people happen to believe that is going to keep its value based on that others will believe the same is indeed opposed to ToSV by putting value in front of utility. No, utility always precedes value, and utility is rendered by the good´s properties. This is true for any economic good, including money.