I did read the David Mack books, and while I did enjoy them, I don't really like that idea of how the Borg could have begun, because it involves human involvement, and that just seems too...coincidental.
Anyway, I always thought that the origin of the Borg was something very similar to the origin of the original Cybermen in Doctor Who.
I think Valykrie and Shawnster are on the right track. We *already* have the beginnings of humans upgrading themselves with computerized cybernetic implants...from limbs to brain implants. And now we have Elon Musk's project to develop a brain implant that could connect to a phone and the internet in 10 years.
And for almost a decade now we have had some scientists experimenting with putting "nano wires" in animal brains, and talking about something called the "World Wide Mind" (google it) were people can literally share uploaded and downloaded memories.
And in Discovery and in the Kelvinverse Trek, we see crew members augmented with cybernetic implants.
So likely, the Borg started out as a race - or group of races - who developed the same technology, and someone got the idea of perhaps connecting more than one person together into a collective consciousness...for whatever reason...maybe just as an "let's see what happens if we" experiment...and then something went wrong.
It could have been an accident, it could have been on purpose. It could have been someone with a nefarious intent, trying to control other members of their species...or by someone's misguided attempt at creating a unified peaceful utopia. It could have been done as a survival strategy (like the first Cybermen trying to survive the death of their world). It could just have been a glitch or a rogue mutated computer virus or a hacker with bad intent. It could have been done on purpose to make better soldier to fight a war. Or to just make better space explorers.
Why or how ever it happened, someone from a race that already used cybernetic enhancements on their minds and their bodies, decided to experiment with making a collective consciousness...for good or evil. And somehow this collective consciousness got out of control and became the Borg.
Though I think that the Borg's drive to assimilate may be a perverted version of the drive that the Federation has to learn and explore and absorb new members into the Federation. Kinda similar to how V'Ger's drive to learn and explore drove it to "storing" who worlds into it's "matrix" or whatever.*
Perhaps the Borg started out as some version of Von Nieuman machine that incorporated an organic component, meant to explore and learn...and it...mutated and became the Borg, and it's drive to make copies and learn and upgrade got out of control...?
*(I don't know if V'Ger downright assimilated/uploaded them or not, but it is strongly implied. But I don't think that the Machine Planet's civilization that upgraded Voyager 6 was the same as the Borg's civilization, because if the Bord had found Voyager 6 floating in space, they would have either ignored it as useless old tech...or they would have assimilated it into their own collective. I think. Either way, I would STILL love to revisit and explore that Machine Planet's society sometime...they are at least a Type 1 or 2 civilization on the Kardashev scale...)
Machine Planet = Isaac's People from Orville?

Wasn't the idea that the Machine Planet helped both V'ger AND the Borg, not that the Machine Planet is the current Borg? Progenitor of both, but without the corrupted mission statements.