An invasion of Japan was never actually on the table. The surrender of Japan prevented Soviet troops from moving into (and most likely annexing) Nanjing, not US troops landing in Japan. The USSR getting Nanjing and the Kuriles back was part of the agreement at Yalta - it was what Roosevelt promised Stalin in return for his agreement to declare war on Japan 2-3 months after Germany's surrender. Stalin later renewed his agreement to attack the Japanese in negotiations with Truman at the Potsdam conference, mere days before the first nuke fell. BTW, at that point, Japan was firmly convinced that the USSR would not only remain neutral, but side with Japan and negotiate a favorable peace deal for them. They were entirely delusional about Stalin's stance, they got told so repeatedly by their ambassador in Moscow, and the allies could freely listen in on any of that because they had cracked Japanese encryption. It was abundantly clear that the USSR declaring war on Japan would have been a complete game changer far surpassing the military impact of the bombings. Meanwhile, the allies had formed a full naval blockade of the Japanese islands, completely controlled the Japanese airspace and had reduced the imperial navy to being almost entirely nonexistent due to lack of fuel. Japan didn't have a chance at that point at all. They were completely fucked and the US knew this.

The entire ground invasion narrative was made up retroactively to justify the bombings, for which Japan had been selected as a target as early as 1943. The US wanted to show off its new superweapon, they decided from the very start of the Manhattan project that they wanted to do that on the Japanese, not the Germans, due to a mix of racism and revanchism for Pearl Harbor, they also saw this as part of a containment strategy against Stalin and they specifically sought out largely unbombed, densely built population centers instead of military installations to make sure that the bombs would demonstrate their full destructive potential by flattening as many buildings as possible.

The idea that Truman's government, which was racist as fuck against East Asians, would have done any of that to help Chinese and Korean peasants, when they routinely painted Japanese as bucktoothed goblins, is honestly kinda ridiculous.

I'm not saying that to write off the reasoning that the bombings may have actually saved lives in Manchuria and other occupied areas as an entirely unintended side effect. That's a different conversation, and a more difficult one than this one. But the potential land invasion is 100% US propaganda and shouldn't even be part of such discussions here. Americans tell this narrative to their school kids for a reason, and it's definitely not compassion with Chinese people.