Once upon a time ... there was the possibility to define hatch-spacing.
This feature was removed in 3.5 due to a change in the underlying technology and was since not reimplemented.
What you can do as a workaround (which we would also need to do internally): create a transparent pixmap with some external pixel-based drawing program
and draw lines in a way so the pixmap can be used as fill pattern and tiled along the filling.
e.g. like the following attached example, which is a 16x16 pixmap