Except there's no evidence that light has slowed down. We know perfectly well why there's a red shift and a blue shift, and it's not because the speed of light is changing.
The theory of the red shift and blue shift is in relation to the slowing of the expanding universe, which will eventually start to collapse back into itself, or so the theory goes.
That doesn’t address the initial high rate of expansion however. That’s just deals with what we observe now from this vantage point.
These light or frequency shifts are relative to the “speed” of light, so it does in fact prove that the speed of light changed, otherwise all light would be the same color because it’s speed- “frequency”- would be the same. We wouldn’t be able to tell which galaxies are further away than others.
Anyways, all we have is theories, and theories aren’t facts or truth, just degrees of them depending on how many other “proofs” there are stacking up in it’s favor.
A truth leads to God however. Amazing how a bunch of Neanderthals who you think wrote the bible came to that conclusion thousands of years ago huh?