I'm not saying you're wrong in suggesting Putin won't use nukes. He may, or he may not. No-one knows what goes on inside his tiny head.
But I do think you are mistaken if you think he won't use them for fear of it resulting in his death. He would very likely conclude that he could use tactical nukes and that the west would never respond with a nuclear attack against Russia, for fear of starting WW3. And he'd be spot on in that assessment IMO. I cannot conceive of any circumstances where the US would risk tens or hundreds of millions of US lives lost, over a conflict over Ukraine, a country not even in NATO. And if the US wouldn't do it, there's fuck all chance of the UK or France doing it.
So the unpleasant reality is probably that Putin can do what he likes, unless/until he's overthrown from within, or he achieves sufficient of his objectives to be able to claim "mission accomplished".