The nuclear threat is Putin holding a gun to his own head.
Fanatics who are genuinely prepared to die, as long as they take a lot of others with them, are rare; The 9/11 hijackers are the exception that proves the rule. Putin is not a religious fanatic in that mould - he doesn't anticipate paradise for his martyrdom.
He won't use his nukes, even if he has any that still work, because he knows that a few minutes after he does, he, his family, his friends (if any), his armies, and their immediate environs, will be rapidly expanding clouds of radioactive gas and dust.
At least, I hope he knows that. And that the world's other nuclear powers remind him of it at every opportunity.
Disagree. He will not use a nuke against a nuclear power. He probably won't use a nuke against a western power. That doesn't mean he wouldn't use a nuke against Ukraine, though.
I am pretty sure that he is on notice from at least three of the other nuclear powers that such an action would not be tolerated.
Certainly the only way to maintain deterrence is to make clear that first use against any state, nuclear armed or not, will provoke a retailiatory strike.
During the Cuban Misslie Crisis, JFK said “It shall be the policy of this nation to regard any nuclear missile launched from Cuba against any nation in the Western Hemisphere as an attack by the Soviet Union on the United States, requiring a full retaliatory response upon the Soviet Union”
The only way to prevent first use by anyone, is for at least one of the other nuclear powers to state, unequivocally and believably, that all such use, against anyone, demands a retaliatory response against the leadership and remaining nuclear arsenal of the nation responsible.