why barnstormers in the 1920s were the way they were, because most of them either learned to fly in WWI or learned to fly from WWI aviators

Have you seen Only Angels Have Wings (1939)? It is one of my favorite movies which I continue to feel absolutely inadequate to describe, but it's not only in clear descent from WWI aviation traditions, at least one character in the cast canonically learned to fly in the war (and you can see it).