Peter is my #1 favorite for a boy. So I'm predisposed to like / love
Petra as well. It's just the sort of underrated classic that I love seeing reborn. It's known, but not overly used. And it has a gorgeous sound. What's not to like / love? :-)
My stock combinations for it are
Petra Vivien and
Petra Linden. From your list I'm loving:
Petra DorothyPetra LucyPetra Margaret (would
Petra Margot be of interest?)
Petra Melody
I wouldn't use a mn that starts with an "r." It's too close to the "r" in
Petra.