Just back from 2 weeks with vet from veitnam who has PTSD. GAC can help some but the crowd is still going to catch up at some point, either in lines or walking or on transportation. We stayed at shades of green, I can say, if crowds are to much and he needs to go back to hotel, this is one hotel that is quiet and peaceful, Disney hotels are crowded at pools and dining area and other areas, shades is quiet and not crowded anywhere, except maybe bus at closing.
Our PTSD person has gone many many times, so he knows and we now know, when he leaves we can still go have fun, for years we tried to go with him, but sometimes he needed an hour and sometimes he was done for the day, which was hurtful to us, we now have a system that he leaves and comes as he wishes but we do what we wish to also, but we text and meet up again when need to. We finally had to realize that even us I the room back at the hotel is sometimes to crowded for him with PTSD, when he needed to collect himself, because one of us would sneeze, or couch or drop a shoe as he was trying to calm down and this would trigger him all over again. So he now does not feel he has to spend every minute with us and keep getting worse, and we no longer feel we have to give up our fun and go back to the room if we are not ready. Talk it over and see if he would prefer you all to keep going whe. He needs to leave.
Just so you know, the fireworks can be heard as far away as I side a room at shades of green, and believe me, he will be able to hear them if he is sensitive, you may not but he will, and since all parks except animal kingdom has fireworks of some kind just be prepared. My ex, was told to start saying his social security number backwards right before the fireworks start, this focuses his brain on something other than his trigger and he can usually get thru it with no problem.
As for crowds, I don't think you can avoid them, we just got back and it was only 5 and 6 days according to touring plans but to me it was very crowded most days, and we did the least crowded day each day. AK was ridiculous and it was a 4, school kids everywhere, abnoxious 5 to 8 graders. Epcot was even worse and it was listed as a 1.1 three of the days we went.
A few stradagies, don't hit rope drop, crowded, since you have a GAC, hit the park after the crowd goes in, 15 to 30 minutes later. Leave before 1 but even if you do not go back in until 6 it is going to be packed when you go back in. This is told not to do, but it was the first time we did it and it worked great for us, we entered MK at 8pm on a day it had EMH, we walked thru the stores and into adventure land, it was empty, did everything, then waited in the back of crowed for parade and then hit splash and BTM by 9 when fireworks went off. Then we went back along the path up to haunted and then into fantasyland by then the park was getting ready to close, we did al those rides and all of tomorrow land,before it closed at 1, 5 minute wait, it was great for crowds.