Parallelising Packer with Ansible
What about using a CI tool? Create a parallel job and the jobs will run parallel. For example, one could use Jenkins:

https://jenkins.io/blog/2017/09/25/declarative-1/


stage('run-parallel-branches') {
  steps {
    parallel(
      a: {
        echo "This is branch a"
      },
      b: {
        echo "This is branch b"
      }
    )
  }
}



but there are other CI tools that could do the same.