This and the other thread has gotten me thinking...and sketching. And so I think I'm going to tackle a more serious effort to design this.
My sources: TOS, TAS and TMP sources as well as period (1920s to 1970s) speculation of future spacecraft both real world and fictional. The key for me is trying to put my mindset back into the thinking of that era.
The only concrete onscreen visual clues we have are the DY-100 class ships of Trek's 1990s and the ringship seen in TMP from approximately Trek's late 21st to early 22nd century.
What we know is that sleeper ships were used for interplanetary travel (within the solar system) until 2018 when some sort of advanced propulsion became available and that most likely wasn't FTL. We know Zefram Cochrane developed a form of space warp sometime around Trek's mid 21st century.
What we don't know---and it's rather irrelevant---is what kind of mission the Valiant
was on, how long it was operational and how long it was contact with Earth, and finally how and when it ended up at the galaxy's edge. We do know the ship was classified as a "galactic survey cruiser." That does suggest extended range exploration, at least within limits of the era.
Now on to the drawing board...
Hmm...should I start a thread?