Multiple issues with saving .xlsx file as a .csv

superbeau 

To create separate csv files for each worksheet you may use powershell , sample is here Convert Excel file (XLSX) to CSV in Powershell , or similar VBA script, or manually: right click on sheet, Move or Copy, check Copy, from destination select new book and from it save as csv. Content will be in Excel window since that's default application for csv files. Modern Excel save all csv sheets into the file if Save As.

If close and open csv file in Excel again it performs default conversions, that was for all versions. In particular, numbers saved as texts will be converted back to numbers (but not only, there are another conversions). I don't know about workaround, the only not to open in Excel but import to Excel using legacy wizard or Power Query. In both cases mark proper columns as having Text type.