Circuit or device for avoiding electrical overload
"Is there a device?" There is probably nothing that you can buy that will do that. You can prevent the breaker from tripping by installing a fast-acting breaker with a lower current, but that just shuts the loads off nearby rather than elsewhere.

Could you design something?" If the loads act like resistors, a device that reduces the voltage would reduce the current, but it is not very likely that the loads act like resistors.

You could design chargers that do what you want, but they would probably need to be interconnected so that all of the chargers can set their charging current according to the number of connected chargers.