An hour to six days is bad. There must be some kind of data ingestion and pipeline involved. Reading a database and generating a formatted xlsx file can be a few seconds to minutes.

An OpenXML library could modify and write to a new file without Excel. Microsoft’s version is usually for C#, there are similar libraries in other programming languages if your computer is locked down. If Excel gives a corrupted error when opening, the XML data can be viewed by renaming the xlsx extension to zip.

Merging Cells: https://learn.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.mergecells?view=openxml-3.0.1

Cell style and background color: https://stackoverflow.com/questions/44839512/how-to-work-with-style-index-in-open-xml