I have yet to see anything that doesn't allow for TMP to be in its own universe.
Yeah. I mean, what sort of contrary proof could even theoretically exist? It's always possible to state "the first three minutes of TMP/Raiders of the Lost Ark/Jumanji/the 1988 Olympics take place in a different universe from the rest", regardless of whether they are somehow distinct from the rest or not.
It has little to no connection to any other Trek series or movie.
That's mostly true of every TOS episode, too. Or of most pairs of Trek TV shows. Connections are not a major or desirable feature in episodic entertainment, and Trek has seldom gone for cross-serializing, save for the 1990s.
For that matter the launch of Voyager 6 doesn't really groove with developments in the Trek universe in the late 20th century. They were launching DY100 series nuclear ships by the 90s, and Friendship One not long later.
Yet they were still launching Saturn Vs in the late 1960s. We don't know when exactly the Trek space program diverged drastically from ours, but it's quite possible it happened after 1977, at which time the only significant difference was that there were six Grand Tourers, not two.
After all, the diverging there is going to be a single event: the inventing of a propulsion system superior to the real-world ones. And it probably won't be a gradual event by any standard. Once the secret is out, every nation or other power will want their own superengine and supership, and we can forget all about chemical rockets and Hohmann transfer orbits.
Timo Saloniemi