A few hipshots:
- The Borg having time travel tech in 2373 might not matter, if it's time travel technology from the future
and not being shared with the Collective of 2373 for reasons of temporal hygiene. That is, perhaps the Queen's troupe is just passing by, timewise.
- We never really learned what the Borg intended to do in ST:FC, unlike in "BoBW" where Locutus claims he wants to be escorted to Earth so that this world can be assimilated. And we never had a reason to think Locutus was being forthright in the episode anyway. So there's little reason to evaluate the Borg performance in ST:FC in terms of planetary assimilation success or failure. All we know of their goals is speculation by Picard.
- Okay, so Picard is in the loop somehow (nanites still in his blood, probably, giving him wi-fi to the Collective). But he isn't so much in the loop that he would learn about the interplexing beacon thing without first confiscating some Borg innards. So his speculation doesn't really count for all that much. Perhaps he was being led on a leash in the first place by being allowed to reveal the weakness in the Cube - thus convincing him that he had triumphed and the Sphere was but a desperation measure or a Plan B.
- The Borg never shy away from battle. They could have flown directly to Earth in "BoBW" already, but for some reason decided to stop not only at Wolf 359 but also at Saturn, Jupiter and Mars, fighting separate battles there before entering Earth orbit. So avoiding the battle in ST:FC would probably not occur to them - why not
gain all the information there is to be gained from fighting Starfleet?