Dear Christian,
Thanks for your updates and I'm following up on the thread with you.
As the Update a group event action needs to update the event based on the id, you may need to also get the id from Graph API.
So it may be difficult to get each variable values from Excel for the web to update to Outlook.
The simplest way to ensure that all events are updated is to create a schedule flow to remove the existing group calendar events and re-add the events from the updated Excel file automatically.
You can use Graph API to get the events' id for Power Automate to delete them.
Uri: https://graph.microsoft.com/v1.0/groups/groupid/calendar/events
Method: GET
Compose: outputs('Send_an_HTTP_request')?['body']?['value']
Compose3: items('Apply_to_each')?['id']
You can create the flow as follows.
Thanks for your effort and time!
Sincerely,
Cliff | Microsoft Community Moderator