You’re right about the public reaction to TMP. Thing is though, to have that reaction in the first place, the thing sold tickets. A lot of tickets. It don’t do as well as Paramount hoped, but it did well enough to make more… then TWOK did profit on a much lower budget and the rest of history.
Basically that’s when Star Trek went from being a steady hit in syndication, much like Irving Allen shows, to being… something else.
Like you though, I’m still not entirely comfortable with the placing of any sort of divide or use of terms such as ‘Classic Era’. As I said, much easier when it comes to Doctor Who, but with Trek, far less clear cut.
If I had to break it up, I’d say TOS - TUC stands as its own thing, with everything from TNG to ENT making up a second branch, Kelvin Movies as their own thing and then what we have now.
But honestly, I find this kind of thinking less than useful, because everyone has their own definition. It’s too fragmented to nail down.