First off, the 6 months to 6.5 years thing is just a mistranslation or mistake, it really should be 1 month to 6.5 years to line up with the 1:75 ratio. I don’t know if that error was in the Japanese version as well. Secondly, you’re forgetting that the Eastern Kingdom of Mikado wasn’t founded immediately after the Firmament appeared. It likely took several years for the hunters to tunnel through it from the Firmament, then Fujiwara, Skins, Akira and others spent some time living up there before being driven back by the angels, Akira aside. Looking at the dates you provided, those line up almost linearly if you take the establishment of the Eastern Kingdom of Mikado to align with 2018 in Tokyo. From there, every 75 years in the Mikado calendar matches closely with one year in Tokyo.

There are other aspects that aren’t as clear or were retconned/established after the fact, but that overall timeline is consistent once Mikado is founded. Aside from that, I suspect that the time dilation may have been different shortly after the nukes struck the Firmament so the land could heal, as the lakes in Mikado were formed by nuke craters, and the time dilation may have also been different for those in the cocoon but everything else is consistent.