Yeah, I've been thinking this works better after reading some of the James Blish TOS adaptions which has the show taking place around that time. It makes so of the weirded things from TOS that never got explained make more sense.
Case in point the planet from Miri went from Earth duplicate to a long since cutoff Earth colony on the usual earth type planet humans tend to colonize.
Right. In Blish's "Miri," the planet was 70 Ophiuchi
IV (although he called it "70 Ophiucus" by mistake), it had been settled over 500 years before, and its architecture was "roughly like that of the early 2100s," which would put TOS in the 27th century. He says they're refugees from "the Cold Peace," a tyrannical era from his novel They Shall Have Stars
, a prequel to the Cities in Flight
series. One interesting thing about the Blish Trek adaptations was the way he worked in references to his own SF -- the Cold Peace, the Vegan Tyranny, Bonner the Stochastic, the Xixobrax jewelworm -- as if to imply that ST took place in his pre-existing universe.