I like CSVI a lot but the lack of XLS/XLSX as export format can be a huge disadvantage sometimes. I use LibreOffice which can handle CSV files very well, but Microsoft Excel works terrible with them. Of course I can use import function but some customers can't understand why and and how it works. They only want to open the downloaded file with a simple double click, and usually they get the whole data in a single column no matter what I set up.
I used PHPExcel for this task with my custom scripts. With that converting CSV to XLSX is just a few lines and can be done as a last step, after I created the CSV. As far as I can see it's deprecated now, but I'm sure there are a pretty good tools for the job like PhpSpreadsheet etc. I'm not sure about the license of them but could you please consider extending the funcionality of CSVI with that?