We are the Borg. Lower your shields and surrender your ships. We will add your biological and technological distinctiveness to our own. Your culture will adapt to service us. Resistance is futile.

Have they ever made good on their threat, and actually force a species to service them, rather than straight-up assimilate all of them?

Yes: the Brunali (Icheb's species)

The Borg assimilation speech succinctly describes the actions that the Collective intends:

We will add your biological and technological distinctiveness to our own.

From Icheb's assimilation, we know that the Borg acquired Brunali biological distinctiveness. In "Child's Play" (Voy 6x19), when Voyager travelled to the Brunali homeworld to return Icheb to his people and his family, we see that the Borg, in their typical scoop-up-cities fashion, had also sampled Brunali technological distinctiveness:

The next part of the assimilation speech describes the targeted group's post-conquest status:

Your culture will adapt to service us.

There's no reason to suspect the Borg of propaganda or deceitful wordplay. Taken literally, this part of the speech does not say that the culture will be incorporated into the Borg Collective, nor that the culture will willingly accept its new role and act in the Borg's interests. The Collective, from its point of view, is simply stating the fact that it can do whatever it wants according to its own timetable.

In the case of the Brunali, the Borg plan called for repeated visits to the planet to harvest biological and technological distinctiveness. If the Borg had considered the Brunali a threat — for being prominent founding members of a strong interstellar federation, for instance — then the assimilation scope and timetable would likely have been accelerated as it was for Earth in First Contact.

Since the Borg valued the Brunali as resources and did not consider them a threat, the Collective was content to let the remaining people exist and adapt in service to the Borg, rather than obliterating the culture. The fact that the Brunali adapted in ways hostile to the Borg was irrelevant, resistance being futile after all.

The Borg don't just assimilate individual knowledge, they also take close account of cultural artifacts such as myths and legends. Most of these they discard as irrelevant chaff, but occasionally they find something useful that they keep, preserve and build upon. A good example of this is their search for the "Omega Particle". We learn in VOY: The Omega Directive that their first encounter was via a series of primitive societies whose main contribution to the Collective was cultural rather than because of their biological or technological distinctiveness.

SEVEN: It began with Species two six two. They were primitive, but their oral history referred to a powerful substance which could
burn the sky. The Borg were intrigued, which led them to Species two
six three. They too were primitive, and believed it was a drop of
blood from their Creator.

JANEWAY: Fascinating.

SEVEN: Yes, but irrelevant. We followed this trail of myth for many years until finally assimilating a species with useful scientific
data. We then created the molecule ourselves.

They assimilate all "distinctiveness" bio & tech to the collective. Borg are only interested in assimilation if a species is technologically advanced with knowledge and capabilities that surpass their own. If the race is an advanced one, possessing inferior bio or techno attributes to the Borg's, even if they are a very advanced species the Borg will not bother. They are not interested in destruction of, or conquering the universe. Their only mission is to become "perfect". One race, one mind, possessing no genealogical, bio-chemical, mechanical, intellectual, psychological, and technological imperfections. Typical suffers of the narcissistic "God Complex" not unlike Hitler's idea of a "perfect" species of man. Both ideals and methods are terrifyingly, unbelievably, remarkably, monumentally wrong on almost every imaginable level.