I agree with Bee about the space, they will need more.
I think part of your problem is that you have an elevated coop. I have various coops I keep chickens in. The one I call my grow-out coop is the only one off the ground. I often put chicks in it straight out of the brooder, usually starting at five weeks old. The behavior you describe is absolutely typical for that coop, but not for the ones on the ground.
My brooder-raised chicks typically do not start roosting until they are 10 to 12 weeks old when no adults are present. If adults are present it gets all messed up but you don’t have adults so it doesn’t matter. I have had brooder-raised chicks start roosting as early as about 5 weeks, some well later than 12 weeks, but 10 to 12 is a good average. Yours should be about ready.
Until they start to roost my chicks like to sleep in a group in a low spot. With an elevated coop the low spot is not in the coop but on the ground. Even if I keep the chicks locked in that elevated coop for a week or more the first time I let them into their run, the first night they try to sleep on the ground under the door. This is one of the few times my chickens are consistent in any behavior, the chicks always want to sleep on the ground under that door the first night.
After dark I put mine in their coop and lock the door. If it gets dark it’s really easy to just pick them up and put them inside. If you have a lot of light down there so they can see it may get a little more challenging. Security lights, street lights, even bright moonlight can help them see. But whatever it takes, lock them in their coop.
I don’t put mine on the roosts but mine are a lot younger. At 12 weeks it could be a reasonable idea.
Sometimes the chicks catch on immediately, they start going in on their own the second night. Sometimes it takes three weeks of doing this every night for them to get the message. I normally have around 20 in a brood. What’s typical is that a few catch on after a couple of days and after about a week the last one is going in on their own, but I have had some really stubborn ones that take three weeks.