[You are spouting dangerous lunacy. Mutual assured destruction is a reality and Putin can do it as well as we can if not better. You are way way way way out of your depth here.]
A Russia completely overrun can come back, maybe even take over the lower 48. A Russia killed to the last, because its leaders used nukes against a peer nuclear power, will never again be anything, except a depopulated province of the Islamized Mongols in the neighboring Stans.
Russian first use nuclear threats are empty ones not because the leaders are nice guys but because at minimum the Friends and Family nuclear targeting plan will take care of anyone remotely associated with them, permanently. Jimmy Carter was a mega dove, but even in his time, retaliatory nuclear targeting emphasized the elimination of ethnic Russians by default, an objective Zbig, Carter’s NSA, made sure to emphasize.
If you love Russia and hate Ukraine, nothing but the destruction of Ukraine and its incorporation into the Russian empire will make you happy. For others indifferent between the two, there is no possibility of Russia using nukes. It did not when it lost half its territory with the collapse of the original Russian empire in the 90s. Why would it do so over a loss in Ukraine that’s not even 10% of that earlier loss, that it did not even own at the beginning of the invasion?
Especially when a nuclear first strike would mean the end of Russia for the rest of time? Russia can come back even from foreign occupation. It will never come back from a pile of ashes. To put this in terms you can understand, Russia will not use nukes for the same reason not even the biggest Ukraine-hating Russophile has tried to take out his Congressman for his Ukraine vote. His fear of the personal consequences keeps his weapon holstered.
You are an idiot. Read Tom Schelling before you start talking about how deterrence works. We have no vital intersts at stake. Trying to dupe Russia out of access to its Black Sea ports, which is what we tried in Crimea, posed a real threat to Russia’s vital national interests, you know, like say China blockading all of our west coast ports. You are an utter fool spouting dangerous nonsense. You are way out of your depth.