USB3/2 hub cascading (reverse hierarchy)
Is your end goal to have downstream 4 USB3 ports and 4 USB2 ports total?

As I understand you have 1 downstream path from your MCU (which is full USB3 w USB2 signals as part of it). In this case you cannot split the USB3 and USB2 downstream paths because they are the same channel. The way USB3 works is it will try to work over the super speed pairs and if that fails default back to USB2. At no time are the USB2 and USB3 SS paths both working and I would be surprised if the libraries/hardware would support a logical split of the channels from a single physical channel.