Unfortunately, the ASG down scaling is controlled by the TargetTracking AlarmLow Cloudwatch alarm. It needs to see 15 consecutive checks, 1 minute apart before triggering a scale down. It would allow you to edit it since it is controlled by ECS CAS. I am trying to find an environment variable to change it but so far, nothing.
The mentioned ECS_ENGINE_TASK_CLEANUP_WAIT_DURATION and ECS_IMAGE_CLEANUP_INTERVAL don't seem to be related to ASG/EC2 scale down.