Thanks for helping think GeoffR
You were quite right that the things in parentheses are ignored (in this case the series), and for a moment I was really happy... Then I realised that I acctually wanted to include the series in the sorting.
In the end I went wild with custom columns in calibre, and I now created a 'sortingcode' consisting of the authors initials, series initials and series index, which I only append to the author field when the books are acctually in a series.
This seems to work, so yay
can finally push all my books to the Aura!