You can start whenever you like - I've been thinking about it for months now, and I'm not even TTC!
I'm afraid I don't like any of the names on your shortlist so far. I prefer Frederica over Frederic though. TBH I'd have thought you were Italian if you hadn't told me otherwise.
I don't think this'll be any help at all, considering we don't seem to share the same naming style, but my favourite Scandinavian names are Kari and Otto. I think you can go for something more distinctly German than you have yet.
Congratulations. I love Frederica, Antonia and Helena (I'm also German by the way ). I'm less keen on the boys names as I find Max and Fred/Freddie a little overused. How about Felix, Quentin or Sebastian?
I am not even pregnant and think about names. I have a DS who is 6 1/2 months and the best part of being pregnant was choosing his name. I would spend hours on this site going back and forth. In the end we chose one that wasn't even really on our radar....
One of my favourite girls names is the name of my friend's German exchange partner. I always thought I might use it sometime. The name is Senia/Senya - not sure about the spelling and haven't come across the name since.
Naming good Germanic names sounds fun! Here's my list:
Alice (it's a name that has served me well! ..although there were several songs with Alice in that used to drive me mad when I was a child e.g. Alice the Camel..) Adelheide (my favourite for our young bean if "it" is a girl! ) Mathilda/Mathilde Ingrid Claudia Erica