Unpopular opinion, but the s31 did what was necessary at least at the time. The Dominion had repeatedly demonstrated it was intent on conquest and subjugation(and genocide). The entire system worked by the Founders ordering the Vorta who commanded the Jem'hadar, take out the head and the rest would collapse.
Odo was able to convince the Female changeling to surrender on the basis of his own forged relationships, but the Federation couldn't have plausibly known that was possible or the Dominion would accept it.
Important to remember the Dominion was never defeated on its home territory, the entire war was against an expeditionary force and local allies and auxiliaries. For such a dangerous enemy, any government would take the sort of action S31 did.
I have to agree. The Founders showed time and again that they were basically Nazis on steroids. At least one planet we know they tried to genocide, and nearly succeeded. And they say their history stretches back 2,000 years, how many species have they completely murdered in that time?
Aside from Odo every Changeling we've gotten to know any bit of their personality seem to hate solids. They're a collective that constantly reinforces itself, they themselves admit there's no dissent in the Link. So we can conclude that every one of them has the same sort of view on solids that the Female Changeling did; which is to say viewing them as utterly inferior and completely expendable.
They also altered or uplifted 2 species into complete subservience, something I also think we can agree is pretty fucked up.
All in all, yeah, I would have been thinking the same thing Section 31 was. Genocide these fucks, in retribution to all the times they've done it to others, and to prevent them from ever doing it again.
And no, I wouldn't trust Odo to be able to change their minds. It's just as if not more likely they brainwash him into hating solids instead considering he'd be a drop going against an ocean.
And given that, as you pointed out, their empire remains pretty intact, it actually does make sense to brink them back in some capacity, even as a big bad. Just sucks it has to be in Picard.
I'd have preferred a series in the original TNG/DS9/VOY style set maybe 20 years after the Dominion War. Show how being on the receiving end of not 1 but 2 wars of aggression, first by the Borg then by the Dominion, has changed Starfleet and the Federation. Perhaps they've become more militarized. A new branch of Starfleet, called Starfleet Tactical, has been established to serve as a dedicated military presence of the UFP. Their standard vessel being the Defiant class, though it's been through several design iterations by this point, would be a nice touch.
And fuck these season spanning plotlines, man. They don't even manage to do them as well as the older Star Trek did. Even in DS9 when we had the Dominion War shit going on heavy the writers knew enough to not make it the sole focus of the show. We'd constantly step away to explore other shit going on. Returning to the Mirror Universe, Ferengi politics, Garak's past, Bashir being a genetically engineered autist, O'Brien's eternal suffering, and yes even the old Trek standby of encountering weird alien shit going on like Tosk and the hunters.
What the fuck sort of idiot gets a hold of something like Star Trek and decides to limit themselves so badly by focusing on a single season spanning plotline? It's just mind boggling.