The best option would be to seek a cease fire and negotiate trying to save as much as possible. I wouldn’t be surprised if Russia wouldn’t be satisfied with territory east of the Dnieper River, guarantee of a neutral Ukraine, and changing the Ukrainian constitution so as to outlaw the Nazi movement.
You are pretty cavalier with giving away another people’s land and condemning half of them to living under the domination of a foreign power. Don’t think that such a Russian victory would end this. If anything, this war has united the Ukrainians in their national identity. After centuries of rule under foreign rulers, they will not submit peacefully again to Russian domination. All Russia would have won would be a continuing bloody insurgency. For their own sake, the Russians need to go home.