Do you assume that the speed of light has always been constant back into infinity?
If so, how do you prove infinity?
If not, didn’t light have to get “up to speed,” and/or “down to its current speed,” at some time?
These are just simple questions that occur to me.
I don’t assume anything, that is what you are doing. Infinity is a number larger than any number you can imagine. In fact it is a number with no specific value. Hence it has no identity therefore it does not exist. I don’t believe in an infinite regression. I’ll ask you to answer the same question. What evidence is there to believe the speed of light is variable?
The present "standard theory" is that the speed of light and all other physical constants came into existence at a given point, which we call the Big Bang.
It is not possible to speak of "before the Big Bang" because time also came into existence then. There was no light.
What I find most interesting is that the Big Bang Theory merely restates Genesis 1:1. "In the Beginning, God created the heavens and the earth."