The sheet access report for us exceeds the 1,048,576 rows so we can't see a full report of sheet access. Perhaps one way to solve this is to create one report for external users and another report for internal users. OR come up with a text file export since excel has limits.
I have a similar problem, except we have 1.7M records. I can't open the sheet using Google Sheets or Mac Numbers.
I don't care how the file is split. Splitting internal/external is just one approach.