Have you considered running the PowerShell to create a site and set the storage quota using Azure Automation and a runbook? If you set up that piece you can then create a Flow with the action Azure--> Create Job to run PowerShell. You can then allow your users or admins to run that Flow from within SharePoint, say from a site request list.
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Thanks for your response. Setting the storage quota via PowerShell in an Azure Function or Runbook is my alternative as Plan B. Unfortunately it doesn't make things easier. So if there is the possibilitiy to do all in the Flow via API I of course would like to do it there.