> Core to this is that the US constitution explicitly enshrines federalism in the tenth amendment (the last part of the "bill of rights"), that explicitly limits the power of the federal government.

I think this is the part that non-Americans fail to understand; we have no central tax authority and that principle is enshrined in our Constitution. Like it or not, so long as the US exists in its current form, so too does our wildly disparate tax code.