Seems to me like you were expecting a movie about the making of the atom bomb, when the movie was clearly focusing on the life of the man who created it.
I went with someone who’s entire knowledge of Oppenheimer was the Epic Rap Battle video and they were perfectly able to follow the entire story and the multiple location and date changes.
I would like to hear more of your thoughts on why you thought the movie was pretentious.
As for the sound, in my theater everything sounded great, maybe just slightly too loud in some scenes. Could it be just a bad setup?
Finally, I did not find the Trinity test underwhelming in any way. However, I can understand why some people might. However this ties into my first point. The story is not really about the creation of the bomb, but how making it affected the man behind it. Could be just that didn’t work for you while it did for me.