An amoeba ccp, ccr... is about 0.8J and I know guys using it as their primary without recognizing relevant disadvantages compared to 1.5J S-AEG. So that could be an option.
Personally I like a full auto as a backup if I lost my stealth advantage. Then a well tuned AEP with a highcap is the same as any other 0.5J full auto gun allowed in Germany but lightweight and easy to carry. Maybe that's an option, too.
As for carrying a short assault rifle with a fixed stock: there is a video on youtube demonstrating how to put it on the back without any swinging movement using a one point sling. If interesting, I could search for it...