If Russia uses a nuke, the rubicon of being afraid of Russian nukes is crossed.
I think the answer to nukes would be NATO giving Ukraine access to unlimited equipment and range, except nukes. The war would change from being contained within Ukraine, to Ukraine having free reign on targets. I would imagine Ukraine would take it to Russia, why wouldnt they, at this point ?
There would almost certainly be a huge increase in attacks in Moscow, and other guerrilla style attacks in other western Russian cities.. Chechen style escalation. Probably attacks on the M-4 at Sivert Donetsk River and E-40 from Volgograd which take out the only good roads in the region, plus the 3 Bridges at Rostov.. that would change Russias dynamic to move considerably, and of course at this point Kerch.
I would imagine it would all get rather more ugly, with Russia using some of its heavier long range missiles to more destructive force on western Ukraine, and probably Kiev, and NATO providing similar to Ukraine to point East.
Russia would need to hugely mobilize, however then I think it would be Putins undoing. Its clear sizeable numbers of people in the “western” parts of Russia dont support this war and are tolerating it. that tolerance may well change to disenchantment, that may encourage those in the east heading to Ukraine, to think their lot might be better going north to Moscow seek change.
However, I dont see it happening.
For Putin to fire a nuke, he has to believe he can survive it. Whilst the west and Russia are not talking, the messages are clear on red lines for both sides.
Remember we are afraid of nukes, but so are they.. thats why we have them.
There will be much fear in Russia that over stepping the redline will have consequence to them… and if we fail to act in response, then the deterrent, is no longer a deterrant to be scared of… so both sides will respect that.
I would expect at most, Russia will got to the very brink, to force a conversation with the US, but the US will talk at the brink. However from now until then its a conversation between Putin and Zelensky, but neither wants to talk to each other, as talking is a disadvantage to both sides, both believe they can win… theres too many pieces left on the chess board for a draw.