I am not sure how relevant this is, but the speed of magnetic flux in metals is very finite.
Yes that's true on a macroscopic scale. But to make some vivid picture. Consider a helix, a charge moves quicker along the helix line than what you would guess if you look from a distance where you only would see a current stream line along the main axis. So even if the internal structure of the particle would move at c, from the outside you would guess that these structures would move at all possible different velocities and I think that the same argument can be made about magnetic flux. What we are discussing are at the limit of what scale the space can take, femto meters for the proton and attometers for the electron.