There are also phenomena such as the natural fission reactor at Oklo that demonstrates that the fine structure constant hasn’t changed over the past 2b years.

Part of the definition of this constant is c and pi. It is very hard to alter one constant without altering another.

https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...

It's circular-ish, but we also see alignment in observable cosmic phenomena that strongly suggest the constants hold (such as the periodicity of pulsars being related to how fast they are moving relative to us, which we can measure via redshift).

Actually, the determination of how far you are looking into the past depends on both the constant speed of light _and_ the accelerating metric expansion of space:

https://en.wikipedia.org/wiki/Metric_expansion_of_space