I am using a VB Script that I got from somewhere on this forum for ages now and have built myself a user object that contains things I usually need. I have used it regularly for large datasets in a similar size (around 10.000 lines of csv)
You can input any text, select a folder from a dialog, enter a filename, file ending and then save the file.
ps: the C# script is so it “remembers” the Folder location that you picked even after restart. Plus there is also a boolean input to append data, but I have never needed that.
Do you mean how to read the CSV file again in Grasshopper?
There is a read file component that outputs the lines. You then graft the lines, then do a Text Split using whatever your separator was. Then normally you would do a Flip matrix and then explode tree. That way you end up with however many columns you had as lists. Sorry, not on my PC right now, but hope that makes sense.
ps: the read file component has a known bug, where it is super slow if you are reading from a file server. But you can also build a super simple file reader using a VB, C# or Python component.