When I was in windows I used to use Total Commander as my file manager. There was a function called Branch view (CTRL+B), which basically showed all the files within a particular directory including any files within all the recursive directories. Basic view of Total Commands I am really struggling finding a linux file manager which satisfies that function. Sometimes I use total commander under wine but on some operations is rather slow. At the moment I am using ranger, but it doesn't have this feature
Flat view of directories
file manager