The unrestricted American attacks on Japanese merchant shipping in 1944 and 1945 were devastating to Japan.
Imports of iron ore, coal, petroleum, lumber, tin, soybeans, rice, etc plummeted during that period.
Japan needed imports of these materials and could not survive without them.
The firebombing campaign against 65 Japanese cities was also devastating and dramatically reduced industrial production.
By July 1945 Japan was crippled. Unable to supply its troops and unable to produce significant amounts of anything.
The Japanese would have certainly surrendered before 31 December 1945 even if the bombs had not been dropped.
But I think the use of them was still a wise decision.
On what do you base your "certainly" assertion, other than wishful thinking?
Even if they did surrender in 1945, there would have been several tens of millions fewer to surrender, those having been killed by starvation and disease.
You have no idea what you are talking about. Learn a little bit and dig deep into the invasion of Okinowa and what the civilians did to avoid not only capture, but simple contact with our forces. Ask the Marines who bore the brunt of the assault on Iwo Jima. The closer we got to the main islands the worse it got.
How you like to have been Truman, and to have faced the American public, in December 1945 (as you propose), or 1946, or 1947 - and have said "yes, we had these really powerful bombs available in August 1945, but we decided to let the war run another year, rather than possibly seem cruel. It only killed another 50,000 or 100,000 of your sons and brothers."