If it actually came down to it, would USA/Russian/China nuke major cities, or is that bluff?

[Since I'm the mad scientist proclaimed by myself]

I question your premise.  When has any nuclear power demonstrated, even rhetorically, a willingness to use nukes against civilians?

Other than August 6 and 9th 1945?

they were both military targets.  Hiroshima was the military HQ for all of souther Japan.  Nagasaki was the most import port in the south and held a lot of military industry.  There was other things at play, sure, but there was military justification for the bombings.

<again, I'm admittedly biased>

These types believe that if there are civilians present then it immediately ceases to be a military target. That's why it's so impossible to discuss anything with these people. I wouldn't even be surprised if they'd say Hitler's bunker wasn't a military target because his secretaries were there.

When you kill 80,000 people with a nuke, it's a civilian target. Ironically the Hiroshima nuke killed more US citizens than any other bomb in history.

Ultimately, if there's one civilian there, some people will say it's a civilian target. Otoh, if there's one soldier there, someone will say it's a military target. Thus, the whole military/civilian targets distinction is worthless. Everyone will have a different opinion. But we are the only ones who have used nukes in war, indisputably.

"Distinguishing between military and civilian targets is completely worthless because some people will split it in ridiculous ways" is a bad take.