Why are they trying to assimilate mankind? Didn’t they use to assimilate races to get technological advances? What can they get by assimilating a bunch of savages which barely survived a world war?
That one’s easy. Humans are an integral part of the Federation. Without humanity in the way their conquest of the Alpha Quadrant would be simple.
The biggest Borg retcon that always and still does bother me, is having 7 of 9’s parents be scientists who specialize in the study of the Borg. The Borg had no interest in the Alpha Quadrant until Q dragged the Enterprise into the Delta Quadrant to mess with Picard. And yet somehow years prior you have people studying them? Makes no sense.