I tried your suggestions and they worked, but it was not exactly how I wanted it to be. So I have been searching for other options and finally found the exact solution I was looking for.
Under
preferences > tweaks I found a tweak '
Specify how SONY collections are sorted'. In the 'edit tweak'-field I typed (the bold text is what I added):
# Specify how SONY collections are sorted
sony_collection_sorting_rules = [
(['authors'], 'pubdate')]
The collections (based on author) on my reader are now sorted by published date, which was exactly what I wanted.