I'm sure she will do fine being speyed, there is a risk with any anaesthetic but as she is fit and well she will be better off being speyed now than having to be speyed when already sick if she develops a pyo. As long as her milk has dried up well I would agree 3/4 months post parturition. I have 3 bitches who are all now speyed both the collie and GWP before their first season, the spinone more recently at 8. She had 3 GA's over the course of 3 months as she fractured her leg, so had to have it plated, then re operated on then she was speyed and she did fine.
At work we advised speying before the first season as the benefits of reducing the risk of mammary tumours in later life is reduced with each season a bitch has. I'm confident the mental maturity of the German hasn't been affected by being speyed before her first season, she would have been a crazy 6ft gate jumping idiot of a dog either way! The breeder said she would calm down by the time she was 3.......she's 6 and 1/2 and we're still waiting!
If you wait until a dog is over a year old before neutering the behavioural benefits like reducing the chance of wandering/humping/aggresion are not always as good. I would have the boy neutered if he was mine but as he is already aware of how it all works

you may not find it has much effect on his wandering but he won't be able to get his daughter