I thought terragrunt was a must have for that kind of deployment.
backend "s3" {
region = "us-west-1"
bucket = "foo-tf-us-west-1"
key = "foobar.tfstate"
dynamodb_table = "tf-lock"
}
https://www.terraform.io/docs/backends/types/s3.html#dynamod...
Their documentation might be lagging occasionally. It's a small team tackling a big challenge.