"The Son'a are known to have produced vast quantities of the narcotic ketracel-white"... So any involvement with the Dominion was before Insurrection. Clearly after Insurrection the Son'a weren't allied with anyone.
The movie doesn't mention for whom the ketracel-white was being produced, though; there is no mention of the Dominion in the war, other than with respect to ongoing negotiations (which probably could not happen after "Statistical Probabilities" and would only resume after the war was unexpectedly concluded).
This is our only mention that the KW might have "narcotic" qualities. There's yer general market for it, then - be it literally as a calming-down substance, or more broadly as a recreational chemical with negative connotations...
The DS9 episode in turn makes no mention on how long the Son'a would have been delivering KW to the Dominion. The alliance might be recent or ages-old. Who knows, perhaps the Son'a who are a refugee species from a refugee species originally were part of the Dominion, and now derive benefits from a contract originally signed by the "grandfather culture". Clearly these Ba'ku folks are technologically advanced, and might actually have crossed from one quadrant to another in their flight from the technocratic-oppressive originating culture. All it takes is time...
Curiously, while there's this urban legend that KW would be difficult to manufacture, possibly even beyond the skills of the UFP, nothing of the sort is actually stated in any episode or movie. So the Son'a are painted as unscrupulous: anybody could do it, but only these guys stoop to it...
They didn't say they couldn't. Indeed I got the feeling they'd be happy. Picard was happy with that idea.
Quite. But Dougherty was being lied to that the Son'a needed the whole supply sucked out of the rings; perhaps there was some truth to it, but the prime motivation there seemed to be Rua'fo getting to make the Ba'ku suffer and die. Possibly
everybody would have been happy with the rejuvenation spa idea if the truth were known; and possibly the spa is there now, only some plot twist keeps it from being all that effective. Say, LaForge's new eyes never took, as we find out in ST:NEM. Perhaps the effect rapidly goes away once one leaves the planet? It would make massive sense for Rua'fo to lie to Dougherty about the effectiveness of the cure, too...
Timo Saloniemi