There isn't any such thing as a "hive mind" unless your talking about telepathy or some other sci-fi garbage.
And I'm going to argue that yes, there are. And why would telepathy be any different from other forms of communication? It would be a efficient way to communicate abstract concepts, but if you can communicate abstract concepts with chemicals or language or facial expressions, what are you lacking that telepathy would give you? And any beings communicating in ways you can't understand is functionally identical to telepathy anyway. They know what others of their kind are thinking, and you don't know why.
Ants, like antmen, when just conduct their daily activities with zero thought. They mill about, farm, carry their young around, all on instinct. Nothing tells them to do it, one antman cannot communicate with an antman miles away, ect. They are for the most part, completely mindless unless they come to a situation where they have to think or use reason, and their mind "turns on" and they have to conduct some sort of business other then the typical stuff them must do.
If nothing tells them to do it, how do they farm and cooperate? Genetic memory? Entirely feasible, at least in a fantasy setting, but I find cultural memory more so. That's how humans do it. Humans learn a new skill or concept, for example farming, from other humans. They then teach it to other humans. And the knowledge survives, and every human doesn't have to figure farming out for himself and, indeed, have any idea about how it actually works. And when you mostly remove individual thinking, you have mindless insects learning from other mindless insects and teaching other mindless insects. Functionally, this is quite similar to the collective knowledge of the hive using the mindless insects to propagate itself. In effect, a hive mind. It doesn't become less magical just because it can be explained.
That "hive-mind" stuff is done to death by other franchises, just make it so that its implied that antmen do very little sentient thinking and alot of laboring guided purely by instict. They were born to shuttle rocks, food, larva around, but if it comes to them making a rational decision, they can function on that level as well. Mostly them just planning attacks on your fortress, or identifying a barrel as something that stores food, or I dunno, knowing how to avoid traps or open doors, or pull levers.
I mostly agree with the intellectual level you suggest. Individual antmen are little more than animals, not exactly stupid, but certainly not smart. Can recognize common objects they have seen before, and operate simple machinery (doors, levers) with a little effort. I suggest that since they're ant
men, they should have hands and be able to use simple tools, like swords or picks, at least if they've seen a dwarf doing it.
I think the difference between instinct and a hive mind is mosly learning. If we have antmen acting purely on instinct, the hive will be unable to react to something the antmen have no instinctive response for, every time. If we have something that functions as a hivemind, the antmen can learn. So that if you get many antmen together and give them some time to plan, they can avoid traps, ambush your dwarves, focus their attack on unarmed civilians and remember which lever operates which door.