Which was another hilarious oversight on the writers part because benamite was only needed for the second Slipstream drive and not the original.
That's complete speculation, they never went into detail how the drive worked in 'Hope and Fear'.
So no, it wasn't an oversight for them to follow the only canon information available on how slipstream drives work.
If you want an actual legitimate oversight with on screen facts backing it up, it was said in Voyager that benamite crystals can be synthesized. In the 24th century it would take years, but surely by the 32nd Century they could have found a way a faster way to do it.
Yes they did, they went through a transwarp corridor in one episode, it was filled with debris from ships which was one reason given why they weren't used often, mostly just by couriers like Book
The courier network was a system of transwarp conduits in the 32nd century. They were extremely dangerous to traverse due to being filled with debris, and thus despite their name were mostly avoided, even by couriers. (DIS: "Su'Kal", "There Is A Tide...") In 3189, Kyheem conveyed a message to...
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and the singularities used by the Romulans.
That's also complete speculation, there's no dialogue on screen saying one way or the other if Romulan warp drives use dilithium.
We know Romulans did use dilithium in some way as they (i.e. Remans) mined it on Remus according to Nemesis.
Non-Canon, but Rick Sternbach's diagram for the Romulan singularity drive says it uses dilithium.
So why didn’t Starfleet outfit all their ships that way then? And why did it only take 5 years instead of 300? And why wasn’t that spelled out on the plaque?
I call shenanigans on that idea.
Then it was one of the other dozen ways ships have intentionally or accidentally travelled cross vast amounts of light years outside of a warp drive.
It's not far fetched at all to say they could of found a way there through some sort of anomaly, subspace corridor, an abandoned alien gateway or some shit.
If the Enterprise-A can magically travel to the centre of the Galaxy or V'GER to the far side of the Galaxy when leaving our solar system, then the Leondegrance can get to the LMC through some sort process that cannot be replicated elsewhere.
and there are a bunch of ways to generate the power for a warp drive without it.
AFAIK nothing with the power generating level of a matter/anti-matter reaction or a singularity drive, so they'd be limited to very slow speeds.