I am sure your help would be appreciated at the shelters . You don't have to aim for SAR or (anything but simple) simple therapy. SAR isn't for the benefit of the dog , is long and complex and requires one handler prepared with many skills and dedication to go to call outs and a special dog .
You would do wonders for the dog , and wonders for the adopting family, if the shelter allowed you to do basic obedience and manners - all dogs , not just gsd. You will get excellent experience .
I know when I was in the working world - a bunch of us "girls" would give up part of our lunchtime to go to the shelter to walk dogs around a parking lot so that they could go pee. Some of the older dogs used to "house" manners would hold on to their bladders and would only relieve outside. Since the dogs were "property of the shelter" until adopted we had to stay in the parking lot scrub area of the shelter - could not leave property. So we devised little back and forth heeling exercises, proper doggy social etiquette , some good manners. Dogs looked forward to the extra personal attention . They seemed to adopt better .
There are lots of ways where some volunteerism benefits the dog, the shelter, the adoptee , and you.