Peter is my all-time favorite boy's name. So
Petra starts out with an advantage with me. But even for its own sake I love the simple elegance of it. It's known, but not overly used. It's one of those buried treasures waiting to be rediscovered.
Love, love, love it.
All of your combinations are great. My favorite is
Petra Eleanor. My two stock combos for it are
Petra Vivien and
Petra Linden.