If the question is how to stop something, TNW's don't do that, they demand a response. That response doesn't have to be MAD immediately, but MAD is certainly closer to an outcome following the deliberate employment of a TNW or NBC.
If the question is to escalate to deescalate, then again it is not the answer, as it absolutely demands a response, and that response has to be such to be a deterrent to the next 2 kopek tyrant that gets their hand on a few rusty old warheads and has malcontent and evil in their heart.
If the question is to defend against an existential threat, then again, it is not the answer, it brings on a response that will lead to that outcome being accelerated.
Russia is losing and losing badly, both militarily, economically and demographically. It seems insane that he has risked all of this for what, to prove Russia is still a force to be reckoned with, a player on the world stage? Its certainly nothing to do with imagined threats emanating from Ukraine or sleights against ethnic Russians. The scale of the response has gone waay beyond whatever was ever justifiable within even your average Russian's mind.
I agree that a single tactical nuke would change very little militarily and would only be used as a means to force a diplomatic resolution. Of course Biden and others have stated (rightly so) that any such use couldn't go unanswered, so where does this leave Russia? How would they in return respond to a NATO likely conventional strike - some direct action against a NATO target or member state? So then we will definitely into Article V territory. Putin - along with the rest of us -
must be uncertain how this will play out and is likely also fearful it could spiral out of control, with devastating consequences for Russia in particular, given they'll likely be on the back-foot in any wider conflict.
I'll say it again - the role of China is critical here. They've stated publicly that any use of WMD is unacceptable. I contend that Putin is probably more worried about losing them as an ally than he his about his forces in Ukraine being swept aside by NATO.
The attack on NS1 was noteworthy in that it demonstrates that, despite his rhetoric, Putin is still wary of doing anything too overt or within the territory of a NATO member state. Its pretty apparent they carried out the attack, but the absence of any response from the west (other than yet more sanctions) must be infuriating for him: "When will they back down??"