I did some thinking about this, after the speculation that Putin would declare victory on May 9, and I realized that at this point, he can’t declare victory, with anything short of a complete and total conquest of all of Ukraine. The problem is that, so far, he’s been controlling the narrative for domestic consumption… but he’s only been able to do that because the soldiers aren’t at home to give their side of the story in person. He’s been telling everyone that the war is going well, but all of the soldiers know that that’s the furthest from the truth. Right now, he can censor all of the communications home through official channels, and whatever gets through unofficial channels, he can claim is Western propaganda forgeries. He can’t do that with soldiers coming home and telling their friends and family face to face. And nearly everyone in Russia knows someone who’s fighting in this war. Even worse are the soldiers who don’t come home: He’s been lying about the casualty numbers, which means that as soon as peace is declared, there are going to be a whole lot of Ivans and Petrovs who are suddenly all officially dead.
He could weather all of that if he got all of Ukraine, and he could show that the war really was going as well as he’d always claimed. Not with anything short of that. So he’s never going to willingly end the war.
Which leaves as the only options for peace, those options that don’t give Putin any choice in the matter. Either the Ukrainians kick Russia out, or internal factions oust Putin, or he dies of natural causes.