On your way to where you live in France, when you stopped in Germany, customs asked you questions about your electronics and you told them what you told us - that you had lived in France for 2 years, and bought all these items in the US a year and a half ago. They asked you if you had declared them and paid import duties, and you said no.
They then imposed import duties on you. Because really you should have declared these items the first time you brought them to Europe, and you should have paid import duties on them. @thorb65's calculation shows that you were charged about what you would have been if you had declared them initially, which is very fair - they could have charged you with smuggling or imposed a penalty. It doesn't matter that no-one asked you about the electronics - it's your responsibility to declare anything you are importing. (I'll assume that "insurance" is just a misunderstanding about what you paid - there is no reason to pay insurance).
If you really did pay import duties when you first bought the items, or if you can show that no duties were necessary, you may be able to clear this up - as long as you can back up what you say: but if you didn't, then this is probably the correct thing. It's a bit harsh, but ultimately legal.
Nationality is irrelevant to import duties: it's all about whether you are importing the items permanently, as opposed to just passing through with them. A German, a Mexican or a Canadian would all be charged the same. It also doesn't matter about whether you are entering the EU through France or Germany - importing anywhere into the EU counts the same. Nor does it matter that you have previously imported the items to another country - you still have to pay German taxes (again excepting transfers within the EU).
Here is a page stating the duty-free allowances that you can bring to Germany. Anything over 430 Euros is taxable, no matter where you are coming from (outside the EU).
If you had imported these items as part of a move to the EU, after living abroad, you would probably not have had to pay duty.