Our GSD is 15 weeks old and I've found a couple of things really helped us with the biting.

1) Gentle and touch commands. Gentle is holding a treat in a fist and as soon as he licks opening the hand to give it to him. Touch is holding out a palm and if he touches it with his nose giving him a treat from the other hand. 2) Food toys. So bully sticks, yak chews, any chew toy which is also food he loves these more than our hands.

When he does get bitey if we say gentle he now sometimes changes to licking. It's slow progress but compared to 12.5 weeks old he's really improving. We sometimes stand and turn away or leave the room if we can but if not we chuck him a toy he really loves (He has a long teddy which has a rattling plastic bottle inside which he LOVES because of the weird noise). Redirection with toys only started working for us over the past few days before that he would not care and just keep going at your arms or legs in shark mode.

Also our pup gets super bitey when he needs to have a poo so maybe check he doesn't need to go?