150K civilians died just taking that small island not to mention over 100K Japanese soldiers and almost 13K US deaths. On the main land things would have been worse. The Japanese military might have been less willing to through their own civilians lives away but that id doubtful. Some estimates actually go as high as 5-6 million civilian deaths with an invasion.

There really is no logic to the argument that we would have killed less people by not using the bomb. We would not have only used planes but we would have brought our ships in close and there would have been massive naval bombardments of their cities. We would have blockaded all their ports and blown up their rail lines so there would have wide spread starvation. The Japanese military typically took upwards of 90% fatalities in defeats and had already lost over 3 million troops. I think the estimates may have been low if anything.

Russia was also going to invade so a part of Japan would have fallen under their control so there was that added bonus. If Truman had refused to use the bomb and 400-10000 American soldiers had died as a consequence he would have been impeached and the democratic party would have dissolved.