Yes no prefect solution.
I am not totally anti cloud no getting away from it now.
Just trying to be as secure as possible especially with garage doors. Since the car sensor is powered off after arriving it should never accidentally fire an open rule.
I also use a battery powered SmartThings arrival sensor in my kids backpack. I had to set these to 10 minute departures to try and avoid false departures caused by hub slow downs.
To set SmartThings arrival sensor depart to 10 minutes you have use the
Xbee3 presence DH