Quantom X

Correction: There is nothing in the movie that suggests that was their plan in the first place. The Borg cube didn't launch the sphere and begin the time vortex until after their cube was about to be destroyed because of Picard's instructions on how Star Fleet should attack. The Borg would no doubt have been monitoring Star Fleet communications and would have known the Enterprise was ordered to stay out of the conflict, thus not expected the Enterprise to arrive with Picard's knowledge on how to defeat them. Thus it is reasonable to assume that when the Enterprise did arrive and Picard ordered the fleet to attack their weak spot, the Borg initiated a Plan B, and then sent their forces back in time. The Borg would have known, especially with their queen among them, that traveling to the past would be risky for their future, also due to the unpredictable chain of events that would have been caused by them changing their own history even, calling their forces out of the Delta Quadrant long before they originally were going to. Thus it is, as Spock would say, logical that their initial plan was to attack Earth with their superior Borg Cube, wipe out Star Fleet and take over Earth with Picard and the Enterprise a long way away from the battle, his ship being the only one with weapons designed to fight the Borg. But when Picard disobeyed orders and the Enterprise arrived at the battle anyways, the Borg had to change their plan.

Quantom X ★

Correction: Picard had shared information with Starfleet. The reason The Enterprise factor is in play here is Picard could hear the Borg speaking and figured out their vulnerability.