Japan surrendered because they lost 50% of their armed force and Soviet Union is going to war with them, not because the US levelled 2 city.
There are more than 2 City that was completely destroyed (Sieverodonetsk, Kherson, Izyum, Kupiansk, Mariupol just to name a few) and already twice the number of losses happened to Ukrainian Army, if that 2 nuke would have make them surrendered, they would have done it a long time ago.
And you are naive to think NATO wouldn't do anything, that's rightly because Russia already had used nuke, NATO would think there are nothing Russia will not do, they are literally crossing the Rubicon, which mean by then it would have been NATO interest to end this war as soon as possible. Because anytime that drag on you are facing more possibility of Russian using more nuke, and that not just applies to Ukrainian War, it also applies to future war. And your response is, NATO would do nothing....Why?
Nuke are a good thing to use for bluff, but when you actually used it, you can't really bluff with nuke anymore.