Microsoft is sun-setting Exchange Web Services (EWS) which we have been using for about 10 years for migrations in favor of Microsoft Graph. So we're porting our code to the next generation of technology.
If you can run imapsync or the Microsoft email migration you can use the same permissions to move calendars for all users server-side with no intervention.
And we've radically simplified our interfaces.
If interested drop us a line via our contact page.