With a direct access to the database you saved efforts to export all needed tables/queries to txt-files which is only an additional step. Further your database contained always the current state from the data - by your txt-files you need extra logics and checks to be sure that the data are always correct. Also you are by direct db-queries more flexible to change/adjust the data-structures (maybe to add fields, combine tables …) whereas the txt-files needed to be (all) replaced. The possibility to use login-credentials could be an advantage, too.
Nevertheless you will benefit from incremental load methods or needs them by a database access in the same way like by txt-files.
My knee jerk would be to use ODBC direct against the database, thus cutting out the extract to text file step.
But your 3 am daily extract to a singe text file, load to a single QVD and load to a single dashboard sounds simple and has done you well for 6 years.
So I would say stick to your text file extract method.