The Borg are hardly perfect, they are too obviously evil looking, a perfect enemy is one you don't even see coming, not one in a scary cube ship that tells you upfront what theor plans are.
The Borg's biggest problem is that they're not pretty. Even if beauty doesn't matter to the collective, shouldn't they realize that getting rid of the tubes, having drones look sexy and changing your slogan from "resistance is futile" to "assimilation is bliss" would attract many people?
Imagine a sleak looking borg ship with 500 silver wearing male and female Seven of Nines that possess fake individuality (it can't be that hard for the collective to have the words "I'm soandso and being a borg is awesome" coming out of a drones mouth) showed up in orbit and declared that you are invited to join them, don't you think many people would line up for assimilation willingly? Of course some wouldn't trust them, but you always find doofuses who'll join the collective. Make a collective within the collective, that allows the new drones to keep their individuality at the start, let them experience all the good parts of being part of a hive mind and let them convince others, let them use their own words, don't force them to do anything. If someone wants to leave the collective because they realize it's not what they want after all just let them go, remove all traces of nanoprobes etc., it will make you look more trustworthy and three others will join instead. Once a sufficient part of the populatio is drones switch off their individuality and finish the job.
It would take longer but you don't have to blow up half of the population to get the other half.