The hub I mentioned in the title is the only device mounting to the SS port, SS03:
View attachment 508879
It enumerates on both ports, but all devices connect through port HS03.
In the screenshot above I have an SSD plugged into a front case connector with a USB C to A cable. It only shows up on port HS09, not SS09 where I would expect it. IORegistryExplorer also shows no connection on SS09.
Plugging the same SSD into the hub in question shows it mounting to HS03:
View attachment 508880
Plugging the SSD into the USB C port on the motherboard connects to the SS06 port at 10Gbps, so at least I've got that going for me.
I was previously running an SSDT I found for the Z390 after I rebuilt the machine; once I discovered the USB wasn't working as expected I tried generating the USBPorts.kext.