What your asking directly stems from two related open ended philosophy-of-science questions. These would be " Are universal constants actually constant?" and "Does the speed of light differ in speed at any point of time in its journey between two points of space in a continuous substrate?"

The answer to both like all philosophy questions is a long hit on the pot pipe and a "sure man, its possible but remains unlikely/over engineering the problem until we have justification through observing it" however I'll give my two cents.

"" Are universal constants actually constant?" " it probably depends on the constant. Fundamental math stuff that tie directly into computations logic and uncertainty precision limits like pi are eternal and unchanging. More physics type constants derived from statistical distribution like the cosmological constant might shift around a little especially at quantum precision error scales.

The speed of light probably is closer to the first one as its ultimately about mathematically derived logical boundaries on how fast any two points universe can interact to quantize a microstate. Its a computational limit and I don't see that changing unless the actual vaccum substrate of spacetime takes a sudden phase shift.

"Does the speed of light differ in speed at any point of time in its journey between two points of space in a continuous substrate?"

Veritasium did a good video about this one. The answer is its possible but currently unmeasurable . so if all hypothesis generate the same effective results then the simplest among them (light maintaining a constant speed during both ways of trip) is the most simple computationally efficient hypothesis among them.