They look at the tub as their safe place. I agree it's time to ditch the tub. If they get too cool they will snuggle for warmth. Good luck and have fun...
I think they would be fine without having the tub at all! They are old enough now to get used to just being in their coop. I would provide a few lower perches and some higher perches so they can get used to roosting. Keep the chicks in the coop for at least 2 weeks before letting them out so they will go back up at night. They are fully feathered now so 60° should not kill them, especially because they'll cuddle if they're too cold.