I need to execute an SQL and save the result in an Excel file (.xlsx) using an automated command line.
Is this possible or is the only option to save the result as text files using the command line options csv= or tsv= ?

This is currently not possible for Excel. I have added this to the list of enhancement requests.

Actually, I found out that using the command line option excel=myfile.xlsx, Query Reporter generates and saves an excel file, but it still insists on opening the file.
On that particular machine where I am running the command, there is no excel installed, so I set up Windows to use Notepad to open excel files. Then Notepad opens the zipped .xlsx file, but as I am running the whole thing in an task scheduler job, the entire session closes down after finishing the command, so the Notepad instance is also closed.
But it would still be nice to have the option not to open excel after creating the file.