For those that are unaware of the idea of "Eternal September", it has to deal with the old days of the internet (before possibly most reading this were using the internet) where most internet users were hobbyists/professionals. As with any person joining an existing large group of people, they would learn the social conventions that this group already had and generally conform to them. An exception to this would be every September, in which a large influx of new users would arrive in the form of university students gaining access to the internet; these users wouldn't know what the existing online culture was, and it would take time for them to learn what the larger group had settled on for interactions.

Eventually, however, the floodgates opened with wider access to the internet and new users outpaced the ability of older users to really get people to conform to the existing internet culture. This was, somewhat jokingly, referred to as "Eternal September", as it always feels like "September" to the older users.

I would posit that the Borg are vulnerable to a similar phenomena; they have a large hivemind and although drones are shown to not have much autonomy, we do see that drones freed from the collective who have been part of it for a very long time tend to initially resent being removed from the collective (even if they would have objected to becoming part of it in the first place, or later objected to having been part of it). Additionally, although we do see a queen to organize the collective, her role seems to simply be that - organization; and that we see her give orders is an indication that the collective at large isn't simply an outgrowth of her personality, but rather an entity of its own that acts largely independently. I think this is backed up a bit when we see Jurati get assimilated by a singular Borg Queen - although the Queen is able to use experience with regards to the technology to control Jurati, she never actually is able to control Jurati's mind and, as Jurati learns how to use the tech, is able to influence the Queen to the point they are practically equals in very short order.

This would suggest that the hivemind is actually what it says it is on the tin, and is predominantly controlled by the will of those connected to it.

An issue arises, however, in that - excepting Jurati's new collective - the path the Borg have largely chosen to take is one where they add new members against their will; and we do see that, unlike a liberated drone who has been in the collective a long time, a liberated drone who was only recently added to the collective will generally retain a disdain for the collective and the idea of assimilation. This suggests that assimilation isn't fully completed when a drone is attached to the collective and their body begins acting towards the collective's will - their mind is, still, somewhat their own and has its own desires at odds with those of the collective. In particular, a desire not to be assimilated/be part of the collective. In the case of a relatively small number of people being assimilated - with even a planet largely being a minority when added to the collective - the Borg would have to take time once these minds are added to the collective to effectively get them on board with the goals of the collective, until these people turn into those old drones who, if liberated, would immediately seek rejoining the collective.

There would be a limit to this, however; if the Borg assimilated too many people at once, these new minds could overwhelm the collective and effectively change its goals, or even dissolve it. This creates a limiter for how quickly the Borg can expand, beyond just simple resources - they must never assimilate more than a certain percent of the existing collective population in order to ensure that the vast majority is in agreement with how the collective should act.

This would explain not only why they're generally picky about who they assimilate - the Kazon would be a waste of the limited number of people they could add to the collective (whereas if the collective just could assimilate anyone then you could assimilate them as canon fodder, why risk a drone from a species you deem better when you can instead use the Kazon?) - it also explains the apparent exponential growth of the Borg. A relatively small collective would only be able to assimilate small numbers of people. A planet sized one might limit themselves to a few starships or maybe a small colony at most, whereas one thats expanded to be a large empire might be willing to take in entire civilizations.

The reverse is also true; and would suggest that the war against 8472 would have been particularly problematic for the Borg. If the collective were to shrink, it can no longer assimilate as many people safely. If you could consistently do so, then you could actually win an attrition war against the Borg, as they can't just assimilate the entire Delta Quadrant or show up with a hundred cubes to assimilate the Federation to fill in for those losses, because these new drones would be the death of the collective. This is why it was so imperative that the Borg end the war against 8472 as quickly as possible (and why they seemed content with the outcome of 8472 running back to fluidic space and not fighting them anymore) - if the collective shrank too much, too quickly, they can't assimilate large numbers of people again. Worse, if they had recently assimilated a large number of people and those people became a sufficiently large part of the collective, they might be able to act against the original wishes of the collective and leave it/destroy it/etc.

Further this could explain why the Borg are more interested in farming the Federation for technology rather than, in most realities, outright assimilating them. They may have the technical capability to send a bunch of tactical cubes against the Federation, obliterate any defense, and then assimilate every world at their leisure. But the Federation may actually rival them in size, and such an event would be very disruptive to the collective. Instead it's best to sit back and get what you can, without taking on too much risk of triggering an Eternal September, and just slowly grow til you can just assimilate the entire galaxy, and "raise the quality of life, for all species", as they say.