The speed of light is a tautology. We define it via how many meters light travels in a second. And we define the meter by the same measure. It’s just the distance light travels in 1⁄299792458

c is a measurable constant, not come unit that is arbitrarily defined. Like Boltzmann’s Constant, or the ground state hyperfine transition frequency of the Cesium-133 atom… it just… Is.

Therefore, it is a useful tool to define units. You claim it is a tautology because we write it in units of meters per second, while the meter is defined based on c. This is easily disproven, as you can represent the speed of light in any unit of velocity. It is a fundamental constant, derivable through experiment without any unit a priori.