Unfortunately, the way things worked out we're going in June. Not ideal, but the planning involves grandparents and work schedules and a few things wen't wrong so we couldn't go at a quieter time... :sigh: Not ideal and I'm a little nervous. The good news is we're going for 5 days, so there's no rush to cram everything in and I can take breaks in quiet places (or the hotel) as needed. Also, we're staying across the street, so I can take my klonopin and not worry about driving.
*Ideally* we will ride Star Tours on a day that I'm feeling relaxed and at a time that the line is low. It's one of my son's two favorite rides, though, and since the other is down all year, I really want to be able to join him at least once or twice.
You're right, the GAC is of limited use for a lot of anxiety issues. Mostly we are relying on a good plan, enough time to be relaxed and pacing ourselves.
If you have a FP for Star Tours, that cuts out the vast majority of the line. If you tell the CM that you have a FP but the interior line is too difficult for you, then MAY direct you to the elevator, which avoids the interior switchbacks. So if you get a FP for Star Tours, you should actually do okay with this ride.
Definitely speak to Guest Services about a GAC. It may not help with many of the lines, but some of them may be better - for instance, I find that the accessible line for Jungle Cruise, which is through the exit, has a walled off area. If it is not crowded, then I have no problem waiting there - If you go to Jungle Cruise during Fantasmic - there is NO ONE there! I always roll right on.
You will definitely be able to do Star Tours, especially if you have a FP.
Fastpass times are not enforced in Disneyland, just like WDW. This means that if you have a FP for lines you find more difficult, you will know exactly when you will be riding it, and you can plan to take it easy right before and after that time. For example, if you know you are going to ride Star Tours at 4:30-5:30, then perhaps you go wander around Innoventions or ride the Railroad before that, and plan on ice cream or a quiet break right after.
I find balancing between the higher stress rides and the quieter rides helps me get through the day.
BTW, unlike WDW, you cannot skip the Stretching Room at HM. It is a literal elevator that takes you under the train tracks into the show building. I suggest either skipping this one, or bringing ear plugs or a tiny LED light or whatever you need to handle it.
I find Tom Sawyer Island a great place to relax during the day. There is room for the kids to run, bathrooms on the Island, and benches spread all over the Island. It closes at dusk, but it really is a great place to go let the kids run and find a place to sit quietly.