>...We know for sure from Truman et al's own writings at the time show that their only concern wasn't the least bloody end of the war with Japan and they wanted to demonstrate the bomb to the USSR, I think that's pretty undisputed history even by the pro-moral-bombing side of the argument.

There is very little evidence of this. Truman's own writings at time (or later) never mention this idea.

This idea that Truman decided to drop the bomb to impress the Soviets was popularized later. It even has an analysis in the Reddit BadHistory subreddit:

https://www.reddit.com/r/badhistory/comments/1z07v8/deconstr...