Admiral Leahy (One of the most senior Admirals in the Pacific War, Commander of the Joint Chiefs):

It is my opinion that the use of this barbarous weapon at Hiroshima and Nagasaki was of no material assistance in our war against Japan. The Japanese were already defeated and ready to surrender. . . .My own feeling was that in being the first to use it, we had adopted an ethical standard common to the barbarians of the Dark Ages. I was not taught to make war in that fashion, and wars cannot be won by de­stroying women and children

Dwight D. Eisenhower (You probably know who he is)

I voiced to him my grave misgivings, first on the basis of my belief that Japan was already defeated and that drop­ping the bomb was completely unnecessary, and secondly because I thought that our country should avoid shocking world opinion by the use of a weapon whose employment was, I thought, no longer manda­tory as a measure to save American lives. It was my belief that Japan was, at that very moment, seeking some way to surrender with a min­imum loss of “face.”

J. Samuel Walker (chief historian of the U.S. Nuclear Regulatory Commission)

Careful scholarly treatment of the records and manuscripts opened over the past few years has greatly enhanced our understanding of why the Truman administration used atomic weapons against Japan. Experts continue to disagree on some issues, but critical questions have been an­swered. The consensus among scholars is that the bomb was not needed to avoid an invasion of Japan and to end the war within a relatively short time. It is clear that alternatives to the bomb existed and that Truman and his advisers knew it

Japan didn't need to be nuked twice to kowtow to the U.S they were ready to surrender, and would have likely immediately done so if the U.S promised that they would keep their emperor (which later did, partially because he was useful in countering Japanese communism). The motivations to drop the bombs were far more motivated by post-war geopolitical concerns on countering the Soviet Union, then by military necessity.

Source

IMO the best and most comprehensive source on the Atomic Bombings is Gar Alperovitz's The Decision to Use the Atomic Bomb and the Architecture of an American Myth, which argues (convincingly) that the decision was motivated overwhelmingly by post war concerns on combating the Soviet Union, and not by a desire to hasten Japanese Surrender. As both General Eisenhower, and Admiral Leahy later admitted that Japan was entirely defeated at the time, and ready to surrender.

Concerning the recommendations by Atomic Scientists, look to James Franck. He was a Nobel winning scientists who fled Nazi Germany to later become a chairman on the Manhattan project. Prior to the detonation of Little Boy, he along with 7 other top scientists, published a report which argued that:

If the United States were to be the first to release this new means of indiscriminate destruction upon mankind, she would sacrifice public support throughout the world, precipitate the race for armaments, and prejudice the possibility of reaching an international agreement on the future control of such weapons.Much more favorable conditions for the eventual achieve­ment of such an agreement could be created if nuclear bombs were first revealed to the world by a demonstration in an ap­propriately selected uninhabited area.

It was part of the Franck Report. There was a project for an unredacted version somewhere, I forget though.

https://en.wikipedia.org/wiki/Franck_Report

sure, but like, it's not like they hadn't done a shitload of test detonations too