Hi Steffen,
Good day to you!
I hope you are doing well. Thanks for reaching out to our Microsoft Community Forum. We will be happy to help you.
Per your description, it seems that you are looking for a method using the built-in features/ways in SharePoint to create a site template without any scripts or Graph API. As far as I know, if you have built the site in the structure you want, then the SharePoint admin can use PowerShell commands to create a site template from this site and add the template to your organization. For more details, you can view the YouTube video.
How To Create Custom Site Template In SharePoint Online
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When you have any updates, please feel free and come back to share them with us. We will continue to help you.😊
Sincerely,
George| Microsoft Community Moderator