The puppy is still very young, if it seems it was abused heavily. If you do not have several hours or more per day to spend with a dog, then it is better find a shelter or find new home. Overall GSD are very demanding dog, and it takes a lot of training and discipline that it would listen.

If you ready invest some time you can try to have dedicated spot for peeing either having fake grass spot or pee pads and use treats to get used to them.

Same time dogs feels human emotion and if you angry, stressed it can cause your puppy to be even more scared as he doesn't feel safe.

When it comes to the scared of leash and other things it just taking very slow, and rewarding him with a lot of praises.

For eating his own poop, he shouldn't be left without supervision, as eating poop is normal for many dogs and they just cleaning up environment.