There are already several standards in place for this. For many decades, the utilities send a signal modulated on the electricity line when cheaper "night" rate

There are already several standards in place for this. For many decades, the utilities send a signal modulated on the electricity line when cheaper "night" rates are available. This was created to increase the night load as the slow power stations struggled with reducing their power output. There was a special heating system designed to use this, which would heat up over night with cheap electricity and retain that heat for daytime heating. This signal could immediately be used for charging control. And of course the signal would be given only when there is more supply than demand. Certainly not at 6pm. But after 8pm private demand starts dropping strongly (which is mostly generated by cooking). Smart meters are also already deployed to offer time dependent rates, those could be used for smart charging and for heat pumps there is an active control channel for direct control by the utilities. All that is already in place. Of course this can be done selectively, so if needed, some cars could start charging at 8pm, others at 10pm and so on.