I tried some testing, @bendigo to see if I could limit the number of columns that download, and the best I could do is a single assignment. This did not eliminate the columns for each of the assignment groups and the multiple columns for each group. I then thought to hide the assignment group totals from the page, which is a view option, but that did not prevent multiple columns for each assignment group to appear. 

I agree that deleting extraneous columns from a downloaded data table is a hassle. It would be great if we had greater control over which of the columns for each item were included in the download. 

From here you have a few options. You might be able to use an API call to download the specific information that interests you, but that's something I leave to Community colleagues with more skill than me in that area. Another option is to report this as a bug to your local Canvas support team, but they might refer you to to a third option, which is to create an idea. You can find out about that process on the page How do Ideas and Themes work in the Instructure Community? 

Creating an idea does not provide an immediate solution to the problem you described, though it is a solution. I’m going to mark it as such and hope that our colleagues in the Community can provide additional support (especially if there is an API option).