How can I manage changes in desired capacity due to scaling policies in AutoScaling Group state?
The desired_capacity in Terraform is marked in the documentation as optional. So with a proper min_size value, Terraform can wait until the minimum capacity is reached before continuing.

The above, with the addition of scale policy can effectively manage capacity without being specific about desired_capacity in your Terraform code. This will prevent you from having Terraform override the capacity in place at the moment of execution.