Whoever asked you to build an automation to import 1.1M rows into an Excel file must be drinking the kool-aid 
You either need to do “Data Modelling” in Excel or even better, use Power BI. You build generic, consistent steps in the “Power Query” editor and use a folder/file combo as the source. This method can work hand in hand with whatever automation that produced the CSV file. Obviously I don’t know the final product use case, so my answer is limited to the info available in this thread.