An example of taxes giving fiat money value isn't so easy but perhaps it is possible. Imagine a society where the government decreed each citizen should pay a head tax using government issued head tax coins. Paying the head tax was completely voluntary, but people who didn't pay the head tax would have their name posted for everyone to see including voluntary government employees such as fire fighters, police, snow plow operators, pot hole repair people, etc. These voluntary government employees would receive head tax coins as payment for their public services. The public display of people who didn't pay the head tax would also be seen by family members, social organizations, places of businesses, patrons of business, employers, employees, etc. In this example nobody would be forced to pay the head tax and nobody would be forced to us head tax coins as currency. However, I believe the head tax coins would be government issued and would be valued as a currency. Would the government issued head tax coins be considered fiat currency? If it were considered fiat currency, then it would be fiat currency that got its value without force.
In this case said organization is not a government (it does not imose monpollies by force) and the currency is not a fiat currency.
I see the point you are making in this topic and agree that if you analyzed all fiat money around today you will ultimately find that force is ultimately the defining feature that gives today's fiat currencies value. However, pursuing my hypothetical scenario further, what if this same said organization also forced people to pay a different tax that was mandatory and that could only be paid in gold and the tax revenue paid in gold was used to fund the enforcement of various government imposed monopolies. This said organization which is clearly a government that imposed monopolies by force could also still issue the head tax coins that nobody was forced to pay, use, or accept. Would these head tax coins still not be considered fiat money?