Right, one ship is supposed to represent their mobile homeworld.
First of all, the mobile homeworld was YOUR idea:
"If it had been the Unicomplex itself coming to attack Earth..."
And now you're ridiculing it? Make up your mind.
And why not? We didn't know how many Borg there were, and the cube was enormous. At least 28 cubic kilometers remember?
The One Cube was not part of a "swarm", no "swarms" were launched from that Cube in BOBW, the way she said the line in Q Who? implied that true Borg force was NOT one Cube yet that's all we saw. It's not a proper conclusion.
You're grasping at straws. She was describing what happened to her people a century ago. The foreshadowing line is merely "they'll be coming". And they came. Prophecy fulfilled. Any implication that there would be more coming was just as subtle as the similar implications in
Doomsday Machine.
A proper conclusion would be finding the Borg guarding the Transwarp Conduit they used to get to Federation space and destroying it, sealing them back in the Delta Quadrant. Or finding out where Borg HQ is in the Alpha Quadrant and destroying it. Or that one Cube was its own Collective unto itself and the other Borg throughout the Galaxy know nothing of the Federation. Not just leaving it be after only ONE ship attacks them.

Okay, let's go down the list:
- They believed the planet killer came from another galaxy, but never found out how it got there and certainly did nothing to prevent more from coming.
- It was obviously a machine built by aliens, but they never found the HQ of those aliens.
- They never found out whether it was the only one.
- They just left it be after only ONE attacked them.
You give the planet killer and every other comparable threat a free pass but remain hellbent on criticizing TNG's Borg conclusion. It's almost as if you're... an unpleasable audience.
In ALL those cases, the superbeings/weapons encountered are always ancient things and one-shots with the implication being that they were just remnants of long-dead folk.
How can there be any implication about the creators if we have no idea where it came from? And supposing it is a leftover, how do we know it's the only one?
"In ALL those cases"? I thought you would have learned your lesson by now not to make such bold, absolute claims, because of how easy it is to be proven wrong. Which you are. Wrong, that is.
Planet killer:
Spock: "I can't help wondering if there are any more of those weapons wandering around the universe."
Crystalline Entity:
Not a long-dead remnant. It was alive, with no implication that it was the miraculous sole survivor of an extinct species. So the rest are still out there.
Kevin Uxbridge:
Picard: "We leave behind a being of extraordinary power..."
And so on.
TNG and DS9 follow the idea of all folks in the Trekverse having the same types of borders and space no matter where they are, the Fed in the Alpha Quadrant, the Dominion ruling the Gamma Quadrant. To follow this story conceit, the Borg rule the Delta Quadrant. If they DIDN'T do that, then the show just gets slammed AGAIN for not following other shows' rules.
What? There have been plenty of powerful races this doesn't apply to. Obviously, since there are only 4 quadrants to go around. Quadrants weren't even well established until DS9. Federation space does not encompass the entire Alpha Quadrant, and it extends to part of the Beta Quadrant. This is by far the most absurd argument you have come up with so far.