How can I hide the main Excel window at startup and only show my user form?

You have apps based only on VBA and user forms. And you want to only show your user form.

 

To do so, configure your XLS Padlock settings as shown below:

 

Then, add this VBA code to the workbook:

Private Sub Workbook_Open()
 Application.Visible = False
 UserForm1.Show
End Sub