Epicycles were brilliant math, as a model of our view of the cosmos. The crystalline spheres were lousy physics, as explanation.

The models are like religion. Once they become set, they can only be fixed, not falsified.

If intergalactic space expands, why doesn't the speed of the light crossing it increase proportionally, in order to remain Constant?

Two metrics are being derived from the speed and spectrum of the same light. Given it is an expanding space and not tired light theory, the speed is still the explicit denominator.

If the premises are wrong, all the shut up and calculate in the universe is just that much GIGO.