This is incredible. I work in industrial automation and I often have to manage / make edits to "point assignment charts" which are essentially Excel Workbooks with 100+ Worksheets, each containing ~10-100 rows of formatted data which define various parameters each real world "point" should have in order to communicate with RTUs / various SCADA nodes.
Sometimes the feedback involves making a minor tweak on every Worksheet. Sure you can muck around with VBA (which is a horrible language and experience), but I often just do it manually to get it over with instead (click click click... very tedious).
C# is my go-to "business logic" language as a developer, and this looks so well integrated, really excited to use it.
Could be a good task for PowerShell and the ImportExcel module[1].
(PowerShell also being a .NET language and written in C#; it has convenient access to COM control of Excel if you can't import the spreadsheet and export an updated version; through $xl = New-Object -ComObject Excel.Application )