Theoretically even with the legacy free version of SM, you can still use the Self-Service Portal and Enrollment Portal for device registration and management as long as your free instance of SM is still active and functional.
But I'm not sure a 100%.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.