IAM roles and Service Accounts have different functions and therefore it is best practice to prevent IAM users from having Service Account roles. For more information on how to use Service Accounts and Service Account Roles, see the GCP documentation.
References:
https://cloud.google.com/iam/docs/service-accounts
In GCP Console -
In Terraform -
References:
https://cloud.google.com/resource-manager/docs/creating-managing-organization
https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/google_organization_iam