Hello Morten Tollefsen
Welcome to Microsoft Q&A Platform, thanks for posting your query here.
Just checking if your customer is sure about moving to Entra ID Domain services?
Because this comes with many restrictions. Ex, they will not get SSO to AVD itself nor to M365 apps, they won't get Intune Management capabilities etc.
You might be able to manually set the permissions on the new share using icacls or Powershell if you know the SID of each user in advance. Not sure how supported it is but it should be possible.
Hope this helps.