[The First Aria]

Basically Prog develops the milk producing structures, which you can't do until you have the basic breast tissue that E gives you

Maybe, but many doctors feel that no Prog is needed for MTF.  They say in CIS Women, it only sloughs of the uterus wall during menses and help with child bearing.  So, IMO, no one knows for sure other than it took my T from 200 down to just 5.