Using USB blocks in an usb-installed QubesOS

Check if you have multiple USB controllers and they are available on the physical USB ports. If you have them then you can dedicate one of the USB controllers to dom0 and connect your USB disk to it and then use other USB controllers to create sys-usb.
Run this command in dom0 to see if there are multiple USB controllers available:

lspci | grep USB

Then check if there are different USB controllers connected to the different USB ports:

You can create sys-usb like this:

But in step 4 use rd.qubes.hide_pci=<BDF1>,<BDF2>,<BDFx> option to hide the USB controllers that you want to attach to sys-usb from dom0 instead of usbcore.authorized_default=0 and rd.qubes.dom0_usb=<BDF>.

And use grub2-mkconfig -o /boot/grub2/grub.cfg command in 6 step in either case.