Having Odo rejoin the Great Link was the more important to the Female Founder than taking over the Alpha Quadrant. She stated so herself during Season 6 Episode 5 "Favor the Bold"
Weyoun: "I must say, you're doing a wonderful job with Odo.'"
Female Changeling: "Meaning what?!"
W: "Meaning that he's always posed a potential threat to our plans, but you seem to have neutralized him quite nicely."
FC: "Neutralize Odo?! Is that why you think I'm here?! Odo is a Changeling - bringing him home, returning him to the Great Link means more to us than the Alpha Quadrant itself. Is that clear?"
Since it would end the war, his doubts about long-term life with Nerys based on his time with Laas, and with the revelation that the Federation went to such lengths to annihilate his people, it makes sense he would decide that he needs to rethink his place in life and that rejoining the Link now is the next logical step.