Any training you do is great. I would not take a dog like this to a public trial though. Not only does she seem like a dog that could not handle it well (nor would she enjoy it) but with her history it's a liability and unfair to others.
You might look into nosework vs. rally. Some dogs that lack confidence (my dog, who is different than your dog, but is still sensitive) because of the stopping and starting. If you have a big dog, there is a ton of stopping and starting involved in rally and it's easy to disconnect from your dog if you don't know the signs. If you know the signs well and can focus on your dog, that is helpful. Nosework is often used to instill confidence in dogs and it's something they do relatively independently and they know how to do it. That is something that reactive and nervous dogs can actually trial in because judges stand back and other dogs and people are not close. It puts a dog in new places and gives them something to do, a job, which is something that can work well for Shepherds. And, all dogs can smell. Nosework just creates some structure around it. It's a very popular sport for dogs that are not good in trial environments around other dogs and lots of people. Tracking is another sport you can look into but it's harder to find instruction and there is more work involved as well as a requirement of fields to lay track.
Dogs change a lot from puppyhood to 2-3 years old. I always say that a dog becomes who they are by three, sometimes 2, especially for females or faster to mature breeds. Often, a dog's personality is less about how they were raised and more about their genetic make-up. It explains why some dogs are fearful despite good upbringings and why some dogs are happy, confident pets despite having awful lives. Did you know either of the parents?