Hi Manichean,
with your first line I get:
Author - Title
with your second line I get:
Author Series_index - Title
Meanwhile I stumbled upon:
{author_sort}/{authors} - {series}{series_index:0>2s| - | - }{title}
That seems to work, but gives
Author - Series - Series_index - Title
I find no way to eliminate the - between Series and Series-index ...