>>45when one clan gets a bias and grabs power
Can you imagine the existence of a type of individual who, when put in charge through some unspecified mechanism, would run things for the overall benefit of humanity, the "greater good", disregarding and even going against narrow personal interest? Are you familiar with the Global Change Game experiments run by Bob Altemeyer?
If individuals don't want to voluntarily fund your project, then they dont need it. Taxing them is the form of partial slavery.
You did not address the large project aspect. Imagine some ancient country with a constant problem of devastating floods. Due to geography, e.g. the placement of rivers, mountains and flood plains, this country is naturally split into some number of regions, let's make it a round ten. Through dams and large scale earthworks the floods of each separate region can be solved or at least hugely ameliorated. The resources needed for solving any one region's problem, e.g. funds, manpower and materials, make the project impossible for any one region, but pooling the resources of all ten makes one project at a time possible, so they can all be done in sequence over a large time period, but not interleaved. The people of each region agree to voluntarily contribute funds and resources to solving their own region's problem because the benefits are huge, so no forcing is necessary, but not to the others' and certainly not if their own isn't done first. Every region is also afraid that whichever other region's project is done first, that region will no longer contribute to the other nine projects, and all ten are needed for each project. Voluntary agreement for resource pooling and for which project to do first isn't reached even after waiting dozens of generations, so the devastating floods continue. But everyone agrees that solving all ten in sequence would have huge benefits in improving the people's lives.
What is your proposed solution for this scenario? Should the floods be allowed to continue indefinitely? Should resource pooling and a project order be imposed by some sort of overall authority, which would solve the problem but would initially go against the wishes of ninety percent of the people? Or do you have a third path?