And to calculate the offset needed to get them all synced up involves calculating time dilation, which involves knowing/assuming the speed of light.
These synchronizations work just as well if the two way speed of light is different than the one way speed of light.
To know the speed of light you assume the speed of light is c, but you're trying to calculate c so all those clocks aren't verified synced.
Just read through the wiki or Harvard's books if you'd like, this is an unsolved "problem" in physics for a reason or do you think no one cares about how fast c is?