How to add a custom field in Civicase that can be retrieved later?
One answer is to create a set of fields, set to use for 'cases', then add your six text fields to that set, then set up a webform that is set to Create or Edit a Case, and display those fields.

That said, I can't quite picture your use-case.

You say "Maybe I just create 6 text fields, each one with an associated drop down select with the funder's targets".

It may help if you can provide a specific example in your question.