can anyone provide information about bus schedules after 18:00 (6:00 pm) from the Changsha airport to the local west bus station

The last bus to the West Bus Station is at 18:00 and after that there is no direct bus to reach the west bus station.
If you still want to reach the west bus station, you can take the shuttle bus line 1 to reach the Civil Aviation Hotel and then change to take a taxi to reach the west railway station. From the hotel to the railway station, it is about 10 to 11km.