That's much better! A great temp solution while you get your hands on other things. Kudos for being open to advice and being willing to change things up for the benefit of the fish
<3 If you see them spending most of their time chilling there in between exploring the rest of the tank, or diving there for cover when spooked, you'll see with your own eyes how important having some cover and safe places to hide is for cories.
Some variation in their food as
@DoubleDutch suggested also a good plan. DoubleDutch will also know the species I'm sure! Knows a lot about cories and very experienced.
Bug Bites, the microgranules, is one quality brand that cories love, and if you can get your hands on frozen foods, they can be great for having a variety of more natural foods. Just don't overdo it with blood worms, they're an occasional treat
Insects like daphnia, cyclops etc are healthier/more natural for them, and keeps them occupied snuffling through and filter feeding through the sand. It's good that you chose sand for cories, even if the colour is a bit bright, and as Byron said, you can mitigate that so it isn't so bright, and it'll look great as a contrast against the wood, plants and leaf litter
I'm sorry about the sickly one.. hoping for you that after the water change, and with a chance to de-stress, she'll rally around. She did seem to rally a bit after your 50% water change from the sounds of it, so maybe test the water again, making sure to really shake the living daylights out of the nitrate bottles. If there's any ammonia or nitrites, or if nitrates are above ten, large 70% emergency water change. Same for if she gets worse or passes away.
Otherwise if tests come back 0/0/<10ppm, I'd lightly feed the tank, monitor from a distance, and hope for the best.
I wonder whether they may have bought a parasite of some kind from the wild, if it turns out not to be a result of stress or water conditions. I don't know how you'd be able to tell for sure though.
Hope she pulls through, sorry you're going through this mate. Please keep us updated.
Darn, if you were in the UK I'd be happy to send you a load of live plant trimmings for your tank.