Only way I got it working was by importing the pre-existing policy into terraform and do an overwrite as already mentioned by @jsimonovic . The full code example looks like this:

import {
  id = "001BF0AC280610B4" # Polcy ID of the pre-existing personal compute policy
  to = databricks_cluster_policy.personal_vm
}

resource "databricks_cluster_policy" "personal_vm" {
  name             = "Personal Compute"
  policy_family_id = "personal-vm"
  policy_family_definition_overrides = jsonencode({
    autotermination_minutes = {
      type  = "fixed",
      value = 30,
    }
  })
}