I don't mind TOS stuff set during the 5YM.
Although, I am beginnng to think there is no way that the 79 TOS episodes, the 22 TAS episodes AND the hundreds of TOS novels could ALL happen in only 5 years.
The way I see it, that's just a necessary conceit of series fiction.
I mean, how many mysteries could Nancy Drew or the Hardy Boys really have solved in their teen years? And is it realistic that any lawyer, even Perry Mason, could represent that many innocent clients falsely accused of murder? Or that Carl Kolchak would accidentally stumble onto monsters everywhere he went?
A certain suspension of disbelief comes with the territory . . .