The one thing we unarguably witness in ST:FC is the Borg attempting something awfully complicated, and using time travel for it.
The former aspect suggests their goal is not a simple one - because they almost invariably achieve simple goals through generous overkill, and basically never go for subtlety in things as mundane as assimilation of a culture.
The latter aspect suggests that whatever they try, they will be capable of trying it again, and again, and again, until they get it right. No need to be afraid of ruining a delicate balance, such as with the above musings, because if something goes horribly wrong, there's always the possibility of a second, completely different attempt. That is, the basic secret and technology of time travel isn't something the Borg have to pass on to themselves through a time loop: in the late 24th century, they will have the ability to build Timespheres no matter what.
The two aspects are somewhat counterbalancing: the ability to time travel would suggest the Borg eventually get exactly what they want, but since what they want is apparently very demanding, perhaps they will give up before reaching their goal?
We don't know if ST:FC depicted the ultimate attempt, hiding the existence of umpteen preceding iterative loops - or if we saw one of the middle iterations where the heroes factually triumphed and the Borg decided to try again a bit differently (and possibly a bit less interestingly).
I think that in no case should we assume that this was an attempt to assimilate Earth. The Borg always do that differently: we have witnessed invincible fleets of Cubes on at least two VOY occasions, and Guinan spoke of "swarming" in the El-Aurian case as well. There is also very little reason to believe a single word of what the Queen said. Nor does this appear to be a standard scouting mission, or standard anything. Arguing that it doesn't meet our expectations doesn't get us anywhere, because our expectations for something this exotic will be wrong by default.