I see it differently. Ukraine should consider a deal where they permanently cede crimea, Donbas and Luhansk in exchange for Russsian recognition for their right to join NATO and a promise never to invade again.
It's a crap sandwich, but as good as it will get. Those regions mentioned are already lost and not returning. Frankly, that deal would allow Putin to go home and say he protected ethnic Russian women and children. Otherwise, Putin is too far in to turn back. If nothing else, he will flatten Kyiv and the other major cities like he did in Chechnya. He cannot take a loss. If he does, he ends up living out his last days in a sort of glorified house arrest like Kruschev and Yeltsin did.
That shouldn't be Uke's problem, but it is.
I have plenty of experience brokering deals where the weak party waives their 1% chance of Glory for a viable chance at a normal life.