Does the medicated feed have the Corid in it, or is that something different?
Read the label to be sure.
Corid should have the active ingredient amprolium.
Most medicated chick starter in the USA also has the active ingredient amprolium.
The Corid package probably has instructions for two levels of dosing: one a "preventative" level (similar to using medicated feed), the other a higher level to treat actual outbreaks.
I had one other chick pass from the coccidiosis a day after bringing them inside and starting the Corid. It was the chick that looked unwell and alerted me to the issue. I'm down to 26 out of 30

.
Today is day 4 of the Corid treatment and thankfully I'm not seeing anymore bloody, runny stools and they are going bonkers being in a small brooder after having more space outdoors.
I completely moved the open air coop/run to a different location since I couldn't think of any other way to 'clean/sanitize' the dirt/sand.
I planned to move the run eventually anyway. The reason I put the run in that location initially was to let the chickens till the soil and remove weeds before planting a garden for them.
Is there anything I should/could do to before planting a garden there? Or should I just change the location all together? I'm not opposed to removing a few inches of the soiled dirt and adding in gardening soil if it would make a difference.
Now that I know coccidiosis is present in my yard any future chicks will be getting medicated feed until they're at least 16 weeks old.
As I understand it, the basics of coccidosis are:
--it is present in almost all soil
--there are many different strains
--chickens usually build up immunity to the strains at your location, if they have small amounts of exposure
--exposure to large amounts or unfamiliar strains (like in someone else's yard) will cause problems.
--amprolium works by blocking thiamine, in a way that hurts coccidosis more than it hurts chicks
--you should not give any vitamin supplements that contain thiamine while treating with corid (because you don't want to give the coccidosis any more thiamine to work with.) The amount in normal chicken feed is fine.
So in this case, I would make sure the chicks are healthy again, then move them right back outside. I would probably keep giving them water with corid in it for at least a week or two: you want them to develop immunity without being overwhelmed. Because you can give corid in their water, and you already have it, I would do that instead of buying medicated feed.
I would not try to replace the dirt, and I would not really try to sanitize it either. Sunshine does dry things out (bad for coccidiosis) and the radiation in sunlight is bad for some kinds of bacteria and other little things (I don't know if coccidiosis cares or not). So letting the sun shine on the dirt might be a good idea-- of course that happens naturally in a garden, even without extra effort on your part.
For future batches of chicks, you could use medicated feed or you could use corid in the water. Or you could raise chicks in a dry brooder (no coccidiosis) but give them a scoop of your local dirt every day or so (small exposure) to let them build immunity before they go outside. If you do that, you might not need to use any medication, although it is still a good idea to keep corid on hand just in case.