Where both sides have viable nuclear weapons said weapons only have a singular value.

They are useful as a means of waging conventional war without fear of losing what you started with. In other words you can invade a country without fear of invasion yourself.

Because the only realistically plausible use is in defense of your own soil.

And this is why Putin is trying to declare the annexed provinces as Russian soil.

Though the power of a single nuclear weapon is usually vastly overestimated it really doesn't take many detonations to substantially ruin a country. Take the US for example. You hit just a dozen major US cities with typical warheads and the US effectively ruined. We wouldn't be utterly destroyed. Most of us would survive, but we would never be the same again.

And this is true for Russia too. Possibly more so. Russia doesn't have very many major cities and they're all concentrated in the West near their farmland.

Ukraine doesn't have nukes so it's a different paradigm in that war. Putin might use a couple of small battlefield nukes to quickly beat down Ukraine. The US, France, and UK will NOT open up a nuke war on themselves for Ukraines sake.

Still, Russia would pay a heavy price. There would never be a return to normal. Russia would be completely ostracized from much of the world for decades.