Yeah, I have PMLE, (PolyMorphic Light Eruptions) which sometimes accompanies or is associated with Lupus.
In addition to the crushing fatigue and flu-like symptoms, I get the "fun" side effect where any skin exposed to the sun for more than about 10 to 15 minutes max will begin to blister. I won't go into details, but it gets gnarly real fast. My PMLE had to be diagnosed by biopsies that were taken from multiple locations after I had intentionally been exposed to the sun, and then began to blister.
Heat will make you feel worse if your sun exposure is kicking up, and I have no idea why. So, in addition to the common sense stuff you probably already know, try to stay as cool as possible.
Basically, you will become Princess of the Mole People (because *I* am Queen of the Mole People, cuz I got here first! LOL)
Dress in light colors - but cover *everything*. Remember that UV light is now your enemy - so any vehicle you travel in should ideally have UV tint on the windows. At home, you can probably get away with sheer curtains over open windows to help cut the amount of UV light bouncing around inside the house, but every little bit helps. Remember that places like under your chin can even get reflected UV light, and can thereby cause symptoms.
You can find UV protective sleeves and clothing all over the Internet - on
Amazon alone there is an amazing selection. I have a harder time with my socks than anything; I am so used to wearing liners and I hate socks, but at WDW, I wear them, because my feet and ankles get extra exposure during my time on my personal mobility device.
You don't want sunscreen - you need sunBLOCK. You can go to a dermatologist and get fancy prescription stuff, but a good SPF 50 should be sufficient if you are careful about keeping it applied on what little exposed skin you will have. I use only "ocean friendly" or "reef safe" sunblock. Don't try to rely solely on sunblock - I learned that the hard way!
Cooling towels, big floppy hats, a parasol (I have a favorite I purchased at the China Pavilion in World Showcase years ago that I carry on every trip) long sleeves, long pants, gloves, socks and a deep abiding joy when the sun drops close to the horizon all await *you* on your next trip to WDW! Welcome to Mole Central!