Not sure if I fully understand your issue/scenario…
If you have your battery set to charge whenever the Intelligent Slot is On, regardless of whether the car is charging or not, you will always get it charging (or, if its 100% full, being idle and not discharging) when you are on the off-peak rate.
If you have an off-peak window till 11.00, but unplug the car at 09.00 and the slot finishes, the sensor will change to Off and your battery charge will stop at that point.
The automation can be set to trigger whenever the slot changes, so its not down to a 30min regular check or anything like that. As soon as the integration detects the intelligent slot is off, then the automation will run and the charging will stop.
Apologies if I have missed something in your scenario… very possible!