I would highly recommend renting an
ECV (from an off-site vendor) and taking it through lines with you. There are a few where you'd need to transfer to a park wheelchair, but the majority of lines you can bring the ECV through all the lines. The ECV will give you more independence because your husband won't need to be pushing you so that you can look around shops or something independently. You will not be the only person at WDW using an ECV and you will not even be the youngest person using one.
You will not need a GAC if you are using a mobility aid.
Tomorrowland Transit Authority & Peter Pan are the only two rides with your level of mobility (not thinking about the causes/pain) that you either couldn't ride or might not be able to ride. If you can transfer at least some of the time, you can ride most other rides. I wouldn't recommend some of them with the conditions/symptoms you have, but there are plenty of rides and other attractions you can enjoy.
Using an ECV or wheelchair is just like using any other tool you're given. I was diagnosed with fibromyalgia when I got my first wheelchair at age 20 (ultimately I was undiagnosed with fibro & diagnosed with Ehlers-Danlos Syndrome, instead). Most of my doctors were completely against it because they had it in their head that I'd get in it and never walk again and they were effected by the social stigma. They were honestly happier with my being housebound and in ridiculous pain when I did get out than thinking about how to make it so I could be active. I've been able to keep walking in general more years because I use a wheelchair (now a power wheelchair) when I need it so they were wrong there. Social stigma isn't my problem, that's something that can only be fixed by societal change so that people stop thinking if you're disabled you're somehow lesser. The best way I can do something about that is by proudly using my mobility aids and supporting things which further making things as equal as possible. The wheelchair changed my life. I went from pretty much housebound to being able to get around. I am positive I wouldn't have graduated from college without the wheelchair and then, later, a
scooter. I managed an extra year of working before I had to give it up because I switched to a power wheelchair. Now it's one of the major reasons I can live anywhere near independently (I'm 28 and lived with my parents for 3 years after I had to stop working, but am now back out on my own even though I still can't work). Do not think of a wheelchair as limiting. I think you may find that a wheelchair (or ECV) is far more freeing than you realize.