Ok, how about this if you’re really set on breaking the behaviour. Take an old brooder or some sort of fairly tight confinement and build a low roost inside it that doesn’t allow for any ground roosting at all in the space. Get them set up on that every night, then once they are accustomed to it take it out of the confinement and then try slowly moving it to the desired roosting spot. Because of the hardware cloth under roost design of my chicken tractors ground roosting is a big no-no. A raccoon or mink could easily grab from underneath…Yes, they are in the coop with Branch, Chiquita and Corona. Branch and Chiquita manage to get themselves up onto the highest roost by themselves. I've tried blocking off their corners with the roost, they go to another corner or just the middle of the coop and pile on top of each other. I'm hoping one night it clicks, but they were almost 2 when I got them and I'm afraid this is just ingrained now in their behavior.
Otherwise, if they have made it until 2 and are all okay, they are unlikely to suffocate each other or such in the ground roosting. You could just go with it. I have in the solid bottomed hospital tractor for some rescues that were too sick to roost on a perch at first.