Find unused VPC subnet in GCP

I have an existing VPC network and want to create temporary small clusters, each with a /29 subnet in that VPC network.
Looks like this:

resource google_compute_subnetwork cluster-subnetwork {
  name          = "cluster-subnetwork-test"
  ip_cidr_range = ""
  region        = var.region
  network       = ""

Currently I manually have to update the ip_cidr_range parameter with a free subnet.
Is there a way to automatically select subnet ip_cidr_range?

Regards, Daniel