Hello and welcome to the forum. I can't add much to what Dottie has said. All the work on walking and reactive dogs makes complete sense to me having lived with one and learned what works and what doesn't. You can read a bit about my experiences on this thread
I feed a raw complete diet and don't feel that diet made any difference in my case. If anything things improved when I started it but that may have been just timing. She is an ex dogs trust dog and they had no information about her past so we were in the dark.
Again I haven't had any experience with GSD's but your issues do sound similar. I found that all the traditional training routes didn't work for us because of all her anxieties. We are at the stage now, where she copes with most things much better than she used to but we are mindful not to overload her and put her in situations which may set her back. I don't think we will get her over her issue with other dogs completely but at the moment she has been introduced carefully to SIL's two dogs and regularly walks with them. She looks forward to her walks and seems to have gained confidence if we are approached by off lead dogs. The other dogs actually seem to look out for her and prevent other dogs from going close by putting themselves in front of her which is interesting to watch. We have also found a field we can rent for £5 where she can be off lead, no other dogs, to do as she pleases. This also gives her an opportunity to feel relaxed outside.
It can be overwhelming at times but as Dottie says, the key is accepting your dogs differences and working with them even if what you are doing is the opposite to what everybody is telling you. Things improved immensely when I started to do that. It sounds like you have put lots of time and effort into helping your dog.
I will often give her a day or two without a walk if she has had a stressful one. In fact I pretty much only take her if she goes happily. She mostly is happy to go out but sometimes she just wants to go for a sniff walk so we do.