There are plenty of options to add another internal USB header. Looks like you already tried one splitter and it didn't work, but you must have gotten a bad one, I've used them before with no issues, Amazon will obv let you return it. You can also buy a USB 3.0 splitter and/or USB 3.0 to internal 9-pin USB 2.0 adapter if that will work, or even run a USB type-A to internal USB 2.0 adapter cable from the rear I/O of the board back into the case. They also make half-height PCIe expansion cards with various types of USB headers.
You can also route a cable from the back io panel inside the case. Not the most idea but:
https://www.amazon.com/9-Pin-Motherboard-Header-Single-7-8inch/dp/B08Q2TLWGV