Again, any "Alternate Universe/Reality" labels will only work in the short term, until someone starts up another alternate Trek continuity.
I don't see the problem with this. We'll manage that when we get to it.
The goal is to find something useful in the long term -- something that isn't tied to the current perception or state of the thing, or the current involvement of any individual, but something longer-term something that will still work 20 or 30 years from now regardless of what might happen in the interim.
I understand the desire to create a classification system that is all-powerful and all-encompassing. I really do. But it just isn't possible. The classification we have now is already fraught with anomalies and alterations.
TOS is actually called just Star Trek, so TOS is just sort of a glorified nickname;
TNG refers to it status as sequel of the first series, but it's inherently bound to obsolescence: what about the next
DS9 unlike any other, has a number in it. why not DSN?
VOY and ENT aren't really acronyms, just shorthands.
I mean, where is the coherence? The robustness of it? The 3-letter rule seems the only constant here.