I just don't see that there's actually much - apart from a strike in Kyiv - that doesn't severely compromise Russian forces, territory, and politics by orders of magnitude more than it does Ukraine. And going after an external - NATO - target....
There's *a lot* of conventional response capability that can occur without responding to nuke in kind. Every asset of the Black Sea fleet could easily disappear for one. Every rail and border crossing that's more than a game trail could be reduced to rubble on the Ukraine side, to maintain appearances.