Hi @Linden ROTH ,
"I cannot update any of the extension fields"
You should use these 2 permissions - Application.ReadWrite.All, Directory.AccessAsUser.All, to be able to update User Schema Extension fields. Currently you are only using User.ReadWrite. Reference: Permissions for User Schema Extensions.
"is there a way to limit normal user update to a single field of the Schema Extension"
For delegated flows, the signed-in user can update a schema extension as long as the owner property of the extension is set to the appID of an application the signed-in user owns. That application can be the one that initially created the extension, or some other application owned by the signed-in user - Update Schema Extension. User with required permissions can change the fields in schema extensions.Reference: schema extension service limit restriction.
Please let me know if you have any questions!
If this answer helped you please mark it as "Verified" so other users may reference it.
Thank you,
James