signs a peace treaty and gives credible assurances regarding its' neighbours
How naive. They've been signing things for years without any intention of sticking to them. And what assurances from r*SSia would you regard as 'credible'?
hen of course steps will have to be taken
"Of course"? Not a chance, and there is unlikely to be any consensus about this.
Your phrase "integrate them into Europe" is very telling. Europe isn't "integrated", nor is there any sort of consensus about r*SSia. They will remain in the cold with no hydrocarbons exports to the west and sanctions firmly in place until the war crimes trials have taken place, demilitarisation and full reparations paid regarding harm caused to Ukrainian citizens and to infrastructure. Plus of course full transparency about their intelligence assets in Europe and the Americas.
a new Russian government.
Has any r*SSian government, new or old, ever behaved with honour and decency?