However you cut this cake, Putin cannot win.
He can stretch it out, sure, but in case he's forgotten, the word is 'stalemate' (at best). Even if he pushes Belarus into joining the war, even if he tries to hang onto the lands he ordered grabbing, they and their breakaway minority russophiles will still be a target forever. By plunging in, he has simply expanded the Donbass quagmire.
Nice try Putin, but no bananas.