Process:
Ensure the Environment Tag of Terraform_User matches the name of the var set in TF Cloud (This is important)
Create a new file called account_(env_name).tf Copy the contents of the template file into the new file Uncomment the template file Find & Replate 'account' with the new env name
Add the new aws account secondary var_set in the terraform cloud Workspace.
Commit, push and apply.
Profit(?)
Should anyone require the credentials for manual work, retrieve them from the SecretsManager Entry in the respective account.