Check the manual to see precisely which slots share BW with which, but I suspect you are seeing the _2 sharing BW with _1. So, you might be able to move the USB card to _4, but that may take BW from _3.

I have a R5E, not a deluxe, but in BIOS, there is also a screen in advanced that allows me to set the BW of each slot.

You can try that, but see above, it may not be possible to have 2 x16, and a x8 and a x4 or x8. Whether or not it is should be spelled out in the manual, but it might require some math to track down all the PCIe channels being used out of your 40 available. Come to think of it, 2x16 + 8 = 40, so that might be your answer right there...