I don't have many suggestions but I know how difficult and scary uveitis is. My moms horse, Teddy, recently lost vision completely in one eye from uveitis. He had many flare ups over the course of 7 years and the uveitis finally took his vision this year. We treat his flare ups with oral dexamethesone, atropine to dilate his pupil, and banamine for pain. He used to get devils claw everyday because some think uveitis is an inflammation issue. He also used to get a Vitamin C supplement to boost his immune system because some experts believe uveitis is an immune deficiency. Now he gets a daily multi-vitamin supplement. Even with his vision gone we still deal with occasional flare ups- I think it only a matter of time before his eye needs to be removed, right now he shows no signs of pain. While he was going blind he was very, very spooky, since going completely blind in his one eye he is much calmer. The vet thinks that when he was losing his vision he was probably seeing weird flashes of light, and blurry vision. She compared it to "looking through a stained glass window". There used to be horse at the barn I board at that was blind in both eyes due to uveitis. He adjusted very well and was able to be turned out with other horses.
It sounds like you are doing all of the right things for your horse. The guardian mask blocks out a lot of light. Teddy used to have a guardian mask too, one day he lost it in the field and we never found it again! Now we turn him out with a fly mask with duct tape covering his bad eye, especially on windy days. I pray that Teddy's other eye stays healthy- I can only imagine how terrifying this is for you to go through.
14 year old Quarter Horse gelding "some horses come into our lives and quickly go, others stay awhile, make hoof prints in ours hearts and we are never, ever the same"