Now things are getting complicated.
The contancy of c is might be an absolute truth, but I think that it is. So my personal truth is that c is constant. I do keep in mind that I might be wrong.
Good science however shows that that is unlikely.
You make a false comparison between things I name that may have been different in the early days of the world and c.
I name things like generations. The time for one generation can be quite different among different kind of animals. We know that because it is like that today. 20 minutes for bacteria and about 20 years for us?
With this information we know that it is possible for the time for one generation to vary.
There is no such information about c. It would make no sense at all for c to vary according to the accepted laws by Einstein, and there is no proof for variations.
Of course c could have been higher or lower in the past, but there is no reason to assume that.
Hence the difference between the things I name and c.
My truth is therefore not subjective either.