The bigger issue that needs to be discussed is that we need to recognize that the current balance of power in the world is unstable. The US hegemon is weakening and in this new normal, the current border configuration is unstable and will be corrected to more stable borders. We should accept this not fight a world war over what is an inevitable adjustment.
You're proposing to let fascists regimes rule the world. Why we even fought WWII? We could just accept the border configuration proposed by Hitler.
Nuclear weapons don't change this calculus significantly, except for the fact that a country with nukes can now defend their borders even with an incompetent conventional force, making expansion into a nuclear power negative-sum. Nuclear weapons do not make it easier to annex new territory as they are practically useless tactically. So there is no scenario where a fascist with nukes takes over the world.
What nukes do is massively raise the costs of intervention by stronger conventional forces. For the same reason it is negative sum to annex territory from a nuclear power, it is negative sum to interfere with the core interests of a nuclear power. If Putin judges Ukraine as key to Russia's survival, he may judge the cost of retreat to be similar to how he would judge encroachment on Russian territory, thus making the use of nuclear weapons rational. What nuclear weapons do is force the recognition of the core interests of your adversaries. There is a stable state where the core interests of Russia are recognized, with any further expansion having negative cost. The world will get to that point one way or another. Endless ratcheting of tension in dynamical systems cannot be contained forever.
So, you propose normalization of nuclear strikes usage by invading countries? Which are committing genocide, war crimes, terror. Yet losing a conventional war because of their utter stupidity and incompetence.