I see the Leondegrance as a bit of a test project with the very real possibility that it was quite possibly a one-way trip.
Vonda McIntyre's movie novelizations have Starfleet sending three transwarp test ships to the Andromeda Galaxy. (At least one, the Magellan, reached Andromeda. It surveyed a supernova.) I wish McIntrye hadn't stopped writing Star Trek novels, because I really want to know what happened here. (However, she would potentially have been writing them under Richard Arnold's Reign of (T)Error, and he'd have killed them.)
Starfleet having a propulsion breakthrough, outfitting a ship with a test drive, and seeing if it works or is viable -- I can see that. Maybe the Leo got there and got back but there were issues discovered with the drive unit that rendered the discovery basically useless.
Maybe there was a subspace slipstream that Starfleet took advantage of and there was only a narrow window in which a mission to the LMC and back would be possible.
Point is, there are ways to justify and explain the Leo and her mission to the LMC.
I agree that the design is anachronistic, though.