The Borg can't inovate, but they know other species can. So I figure that they send less than their best against non-assimilated species, learn from those encounters, and in the case of the Federation, those defeats, and then, when the Borg believe the species has reached the point where they can't inovate anything better, they unleash the might of the full Collective on them. They learn more from the Federation that has created numerous weapons designed specifically against the Borg than they would have from the Federation that hasn't had to come up with these ideas to fend off the Borg.
I still think the Borg were mostly obsessed with Voyager, and in particular, Janeway. The Queen eventually developed a bizarre fixation on her.
Conceivably, under normal circumstances, it only takes one cube to assimilate a world.
Attempt a standard assimilation again (minimal risks) & in the event of a failure (Which was imminent) have a fall back plan to invade them at an earlier time in their history
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