The more I think about it, the “Destiny” explanation is just as valid as any other theory, despite it coming from the novels. None of the other theories are canon either, and the “Destiny” origin (it’s really more than a theory, it’s an actual explanation) actual ties to come canon characters (won’t elaborate for those who aren’t read the series, but @Michael Benucci posted a link that gets into it in depth). “Destiny is actually a fantastic trilogy that would’ve been amazing if ever adapted to the screen. High recommend if you haven’t read it, and it more than earns my “head canon” origin for the Borg.