Hi , don't forget that in general a german sheppard not matured out "between the ears "before arround 2 years old , so build your dog , increase pressure step by step ; your decoy should be able to "read"your dog , and don't cross the border with him
I would work on bite development separate of normal bite sessions. It does sound as if there is a confidence issue OR it could simply be more of a decoy issue, not allowing the dog to take a “good” bite before they start moving around to aggressively. Alternate between back pressure and then letting it off and stroking the dog’s body forward. Praise the dog when and only when it has a deep full bite. Also, try alternating between prey bites to short burst of defense being put on the dog, prey by the decoy, quick low amounts of stress by the decoy and back to prey mode by the decoy. It might also help if you start by getting into the fight along with the dog and slowly start moving out and away from the dog and decoy. So that you move close then ease away and back again from another angle until you can walk in a semi circle behind the decoy and dog and finally out of sight of the dog. This should assist in the dog’s dependency on you being there which to me in an indication that the dog is not fully confident in itself. Back tie the dog with you close to but behind the decoy and again move in only to encourage the dog to take deeper bites and not use its front feet to push against the decoy, wrapping the front legs around the decoy is a great thing by the way. All of these should help build confidence and a fuller deeper bite.
