Please tell us where (and how, if necessary) we can change the default workbook.
If that is not currently accessible to users, please add it.
I have tried to create my own default workbook in the Templates and Startup folders, both with and without a file extension, but all to no avail.
Word allows us to create our own Normal.dotm in the Templates folder and that gets used for word defaults. Be nice if Excel did it in a similar way.
This page (hosted by Ron, and Excel MVP) shows how you can do this on Mac Excel.
Version 16 (fast insider build) broke this functionality. It's been at least six months since this worked.