Is this possible? Eg, I would like to have Parent Category > Subcategory 1 shown in the top menu, but then further subcategories displayed in the sidebar menu.
This do depend on your template, but if its fairly standard you can set your top menu to only show 2 layers of categories, from the admin via:
System > Configuration > Catalog > Category Top Navigation > Maximal Depth
I hope this helps :-)
If still needed, you can try our Advanced Menu extension.
Perhaps, you will find the required cofiguation there.
Magento 2 Layered Navigation Extension will help you display your website with more subcategories in the sidebar as you want with its vertical filter feature. Plus, selecting multiple values is available, which makes searching around your website will be much easier than ever. Sound cool?