The Borg should have been 100 times the threat they posed during the entire series.
From "thousands of planets" they assimilated, at least ten would've been evolved beyond our imagination, giving the Borg knowledge and technologies beyond our ability to divert.
I don't know that. The more advanced a species would be, the more the Borg would gain from them, but also the more improbable it would be the Borg could defeat them in the first place (that is, assuming that the species in question doesn't want to be assimilated). For instance, I don't think the Borg are capable of assimilating the Voth civiliziation or cizilizations advanced even more than that. So I would postulate that there is a kind of upper bound on the technological capabilities of civilizations that they can assimilate, and that the gap between that upper bound and the federation's capabilities is apparently not unbridgeable (even if our heroes succeed only with unorthodox approaches and lots of risktaking).