Reddit mentions of USB 3.0 to 2.0 Adapter Header Cable Converter USB3.0 19 Pin Female to USB2.0 9 Pin Male convertor Motherboard from Intel ASUS Gigabyte MSI Ports(19Pin Female to 9Pin Male)

Back to one of your initial thoughts, connect this little guy to an open header to either your motherboard or add-on PCIe. If you feel more comfortable doing so. Just be sure to verify the individual wires are ligned up to each other's respective pin position before you power up the machine. https://www.amazon.com/Adapter-Converter-convertor-Motherboard-Gigabyte/dp/B07L9RWRMM/ref=mp_s_a_1_7?keywords=usb+3.0+to+2.0&qid=1563837188&s=gateway&sr=8-7

This might be convoluted, but to adapt usb 3.1 to usb 2.0, could you use two different adapters? Like, 3.1->3.0 and then 3.0->2.0?
Maybe a combination of these cords?

https://www.amazon.com/Adapter-Converter-convertor-Motherboard-Gigabyte/dp/B07L9RWRMM/ref=sr_1_4?keywords=usb+2.0+to+usb+3.0&qid=1572354854&sr=8-4

https://www.amazon.com/Goliton-Front-Header-Extension-Motherboard/dp/B078Y2JNXN/ref=sr_1_3?keywords=usb+3.1+to+usb+3.0&qid=1572354770&sr=8-3

Your motherboard doesn't have it. It only has a USB 3.0 port which is a different port altogether. It's usually occupied by a cable that connects it to the ports in the front of the case. If there's nothing in that port, you can use a USB 3.0 to USB 2.0 port like this:

https://www.amazon.com/Adapter-Converter-convertor-Motherboard-Gigabyte/dp/B07L9RWRMM/ref=sr_1_7?keywords=usb+3.0+to+usb+2.0+motherboard&qid=1570512542&s=electronics&sr=1-7

If it's already being used up then you would need something like this :

https://www.amazon.com/Adapter-designed-motherboard-external-connector/dp/B000V6WD8A

You would have to route it to the back of you case somehow and connect it on the back.

Just to make sure I got it down right. https://imgur.com/a/YI0p7IJ

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First image is the old bluetooth I need to unplug. I assume I can just unplug it and leave it inside?

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Second image is the USB header in question?

Looks like this may work https://www.amazon.com/Adapter-Converter-convertor-Motherboard-Gigabyte/dp/B07L9RWRMM/ref=sr_1_7?keywords=internal+usb+header+adapter&qid=1568729305&sr=8-7

Try using a micro usb cable and running the cable directly to an external usb port. It's ghetto but a possible alternative.

Or if you aren't using the 3.0 header theres this https://www.amazon.com/Adapter-Converter-convertor-Motherboard-Gigabyte/dp/B07L9RWRMM/