Well, life can be so simple sometimes. Can confirm my instance ID is in fact used, based on `/run/cloud-init/.instance-id`. I'll add to the tracker that this behaviour is not really intuitive. Thanks again!
There's a known issue (https://bugzilla.proxmox.com/show_bug.cgi?id=1739) where "each time you update the cloud-init configuration, or manualy regenerate the image, the instance-id is changed."
To ensure that `instance_id` never changes again...