Is there a particular reason why you need a reboot at exactly that point? That seems like it wouldn’t be really necessary. MAAS reboots the machine after it’s installed, but if you are installing from infrequently-updated images and want to run updates and reboot afterwards, then doing that with custom cloud-init metadata at the end of the MAAS install (i.e. before model is created, machines are added, and juju agent is installed) would make more sense to me.