If I’m not mistaking there are alreaey some providers that offer this.

Certain models (e.g. the ID.4 let you switch on an option that maintains the 12v from the main battery. You might even be abke to keep the car plugged in and start and stop charging sessions with your app. But at 80% charge, without any connections from the app, I’m sure the ID.4 can supply the main battery for months.