My experience with changes to MAAS post-deployment requires you to make the changes to the deployed machines manually until you choose to rebuild them via MAAS mechanisms. Cloud-init should only run once on the host when you initially deploy it, so changing cloud-init data would require a re-deploy.
That’s what I did but was a bit hesitant because every cloud-init generated conf file is plastered with “do not make manual changes changes will get overwritten at boot” but if you say it only runs once at instance create then we’re good