You are repeating Russian talking points. "The West can't do anything to us or we will nuke them." This is simply not true. Actual experts, meaning retired generals and the like, try to tell us that this isn't true.
If Russia nukes the United States, the United States will probably nuke Russia. This is what MAD is all about. The only point at which Putin would push the big red button is the point at which things can literally not get worse for him. If he is losing the war but still alive, healthy, and in charge, then he will have nothing to gain by pushing the big red button that guarantees his own death in about 30 minutes.
You say "we can't cross X arbitrary red line, or Putin will nuke us"
Ok, flip that around, imagine you are Russian: "We can't nuke the US or they will nuke us back."
See how that works?