The author,
Astrid Lindgren, got the name from a lake, Juronjaure. Of course
Astrid wasn't the one who created the name, but it's thanks to her the name has become so popular here in Sweden. It's very popular here and has been for years but I don't know someone personally with the name, but there was a girl in my old school named that.
I don't think
Ronja needs a nickname.
Of yours I like
Ronja Sofia and
Ronja Cecilia.