If we had to invade the Home Islands of Japan, we would have pretty much had to kill everybody in order to subdue them. I have a hard time seeing how the Bomb was the worse of those two options.
You are correct. The casualties would have been very, very severe. I have seen casualty estimates in the range of 20,000,000 for Japanese civilians. This is partly because if the casualties for Americans reached very high numbers, their blood would be up and the war would assume an absolutely pitiless character. It would truly have degenerated into a “kill ‘em all and let God sort ‘em out” type of struggle. The remainder of the Japanese population would have been reduced to paupers in an utterly ruined country.
When someone argues that America could have conquered Japan without the use of the A-bombs, I point out the estimated Japanese casualties and ask them if they really wanted that many Japanese to die.