I can’t envision any war where you come out in the toilet and can be counted a winner. Russia lost when they were dropped from the world economy.
In theory, though, they certainly could always drop nukes on Ukraine and simply move in to overtake the sizzling flat wasteland that remains. But I think that if there was a goal to the war, it’s to teach the Ukrainians that they need to do what they’re told to do, by Russia, and that insubordination will not be tolerated. For that, you need them to mostly stay alive, to repent and to serve you later. That limits how completely destructive Russia is likely to be.
That said, Russia has already lost and they’re at the point where there’s probably not much that the world can still do to them. Once they do claim victory over some particular territory, they might as well nuke Kyiv or drown it in sarin gas, or whatever, just as a final gut punch, before calling it a day. What can the West do to them, still?
Likewise, if Russia’s position falls back too quickly, and Putin looks like he’s not even going to have won the territorial conquest part of the endeavor, he might throw the nukes as a measure to save face.
The long term strategy for peace is to keep the war going, without either side being confident of a win nor a conclusion - it just needs to keep looking endless. And Russia needs to have an out where they can regain some of their economic connections to the rest of the world and still have some land gains. If they are heading for the door and they’re still looking at complete isolation, there’s nothing to lose by being completely immoral on the out.
That said, Putin has an increasing habit of only ever taking a helping hand to gain the opportunity that he can bite it and show how stupid he considers all the care bears, legalists, and humanists of the world to be.
It’s unlikely that there’s any happy ending in any of this, with Putin leading Russia, because of his personality. The Duma really should remove him - for their own sake, and the sake of the people of Russia.
It’s okay to accept that you were wrong, and to start doing the right thing. Maintaining face isn’t worth depriving your children of their futures in a land of peace and prosperity.