Even if it may not be a total "ensure every square mile is in range of blasts or fallout", and even if maybe not every single person in either country dies, it'd likely quickly escalate into humanity's worst catastrophe (so far...).

I could potentially see tactical nuclear weapons being deployed in theaters that are far away from any superpower's homeland, though, as almost happened several times during the Cold War.

I don't follow this. Russia is driving tanks and trucks and flying planes over to Greece. Ok, now NATO comes in with planes and tanks and bombs some staging areas and factories in Russia prior to a land invasion. Ok, Russia Nukes who? Why? They are in Greece, nuking some point in Europe is just going to get retaliated (like Moscow and somewhere else) from anonymous submarines while the land forces stream into Ukraine and eventually Russia. Maybe he's got super intel and is able to pinpoint nuke land forces that will stall the liberation of any area for weeks or months.

Putin would be shitting himself because he would have 2 options. NUKE THE WORLD and kill himself, because he doesn't get to keep Greece and it's going to end in one of those directions, either way.