Keeping their biological side is not subconscious; they plainly do it by design: they approach perfection by assimilating other cultures' creativity. They don't practice creativity themselves per se. Your bottleneck appears to be at the location where you ascribe your own notion of what they should do in place of their stated goals.
so in essence their view of perfection is flawed. are they aware of that? though they keep their biological bodies, they do nothing to unlock the potential of those bodies even though they have the tech. i'm sure there are plenty of species that they've assimilated with amazing biological abilities. one that comes to mind are vulcans. they could release the potential of their minds, but they don't. they don't really elevate themselves. they're doing more of a diagonal rise rather than a vertical rise they have all the tech they need to accomplish this, but they don't. unlock biological potential and augment it with electronic technology. don't you think that's much better than just taking a regular body and augmenting it? which one would you say is more "efficient" or "perfect"? i doubt you'd pick the latter cause that wouldn't logically make sense. so why are they not doing that? do they not realize that their methods to gaining "perfection" and "efficiency" is very "imperfect" and defies streamline logic? if they don't realize that then they really are not following their directive.
also, if they assimilate "creativity" from the outside, doesn't that mean they do so in order to utilize it? if not, there's no logical reason to assimilate it.
i'm saying that their actions don't seem to FULLY correspond with their goals. am i the only one who thinks that? sure they somewhat follow their goals, but not to the best of their abilities, and they are capable of so much more and have the knowledge and tech to elevate themselves beyond what they are now. if they are thinking more machines as we have seen they either should be follwing their directive fully or not. there's no somewhat.
but i think mr. spook 7 had the best answer where the borg really don't make sense. they're just there to be meneacing even though they're not working as intended because the creators probably don't think that people will think about borg function as much as just go "ooh ahh" when they see them lol.