Glad that you are seeing improvement. I think that some people use Corid with Toltrazuril sometimes, but I don’t think that sulfa and Corid are used together. Here is a link for toltrazuril (Endocox:)
https://birdpalproducts.com/products/endocox-powder
The vet could also do a gram stain for necrotic enteritis. Amoxicillin and other antibiotics can treat that. Aqua Mox is available online. The sulfa may also, but there are others as well.
The maximum Corid dosage that many of us recommend for outbreaks is 2 tsp (10ml) of liquid Corid or 1.5 tsp of the powder Corid per gallon of water for 5-7 days. If a chick is still sick after 5 days, do an additional 5 days at 1/3 of that dose.
How are you mixing the Corid? Can you give her some undiluted Corid 0.1 ml per pound once or twice a day in addition to the treated water? Sulfa antibiotics could possibly be better for her if you can get some quickly. Those include Albon, sulfadimethoxine, sulfa trim, and SMZ-TMP.