Sloan was interested in securing the safety of the Federation. Once it was discovered that the Dominion was lead by the Founders, Odo became an obvious target for Sloan. The only Founder that Sloan could get to was Odo. The best chance for infecting them was through Odo. Maybe the odds were long that Odo would ever pass it on to the Founders, but it was the best possible route.
And I don't recall Sloan ever caring or being interested in Sisko, just Odo and Bashir (once his genetic enhancement became "public" knowledge). Bashir would clearly be an incredibly useful asset for Sloan, which was demonstrated in the episode "Inter Arma Enim Silent Leges". Despite the fact that Bashir was trying to work against
Sloan the entire
time, Sloan's plan came off without a hitch! If that doesn't demonstrate that Sloan doesn't need an entire organization of Section 31 operatives, I don't know what would! Sloan managed to make people do what he wanted and needed without them even wanting
to help him. What need for other operatives, then?
But, hey, when it comes down to it, yeah, the writers took Section 31 in another direction. I'm just saying the canon would easily support an entirely different backstory for S31, one that would be far more plausible, and would prevent fans and writers from pinning all the sinister and sneaky stuff done in Trek
on a shadowy organization that managed to keep itself secret for centuries.