I am trying to copy all items from Calendar list in each sub-site to the Calendar list in root site.
Here are my thoughts. Is it possible to make a CSOM PowerShell script for this?
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The structure seems to be fine and here is a sample script to copy list items between sharepoint lists： SharePoint Online: Copy List Item to Another List using PowerShell
And of course you can have a check on the OOB approach using Calendar Overlay. How to show a calendar from a sub-site on the home page