Interesting post and gotcha - herding chickens is not a thing! I wanted to share a few thoughts on your post, just my experience for what it's worth. Firstly, your chicks look great and it sounds like you are taking fantastic care of them! In my experience, considerate husbandry pays dividends with healthy birds and good production. The only concern I have with sand is that it is so heavy...my little coop trays probably aren't built for that...My hens already know how to roost, so they don't stand on the tray...I think I read somewhere how to teach the chicks to roost up on the perches, but I haven't had to do that. I guess my point is, my hens sleep up on the perches in the roosting area and their poop falls directly on the metal tray. Then I just pull the tray out daily, scrape it to my compost area, then return the tray. They don't really lay on it...
Another note is that, all birds "dust" as a natural behavior, and sand is great for that. (I mix Food grade Diatomaceous Earth with the sand for my pet quail... which helps prevent lice, and plan to sprinkle some in the egg laying area of my hen coop.) If you place some sand in a small, shallow wash tub, I guarantee your hens will take a sand bath. This is a natural behavior for them, they use it to remove rancid oil from their feathers. It's important for their well being...They distribute the sand through their feathers, then they will distribute fresh oil from their preen gland, which all birds have, to protect their feathers. This is how and why birds are rain proof, actually it's very important to allow them to do this. My hens don't need a sand bath because they are free range and just make their own dusting area in the dirt...
Oh one other thing is, when I got my adult hens they had never lived in a chicken tractor before, so they didn't understand they are supposed to sleep up in the roosting area. I put a nice little dish with some canned corn and black oil sunflower seeds up in the roosting area. When one found it, the others came up to check it out and I locked them in with it overnight... However mine has a light in it that we installed, so if you don't have a light you could use a (battery) lantern or flashlight if it's too scary and dark for them to go in....Then usually one was too scared to go in, so I just guided her in with a combination of throwing bits of corn on the ramp as a lure and then using a rake to guide her up the ramp, then locked them in.
I believe it is better for them to sleep in an enclosed area, and it's worth it to take the time to help them develop these good habits now...Thanks for your interesting post! Good luck!