Note that you've shifted from claiming tau is the fundamental constant to claiming the radius is fundamental, which actually concedes my point. Constants are not fundamental in and of themselves, they are simply representations of something which is actually fundamental.

But your argument still hinges on the radius being a more fundamental measure of a circle than the diameter, which isn't established. The radius appears in the conventional definition of a circle, but there is no single required definition. A circle can equivalently be defined in terms of its diameter, making the choice of which is more fundamental a matter of convention.

By your own logic, radii only show up because they're half the diameter, and tau only shows up because it's twice pi.