I successfully used Beagle USB 12 and 480 https://www.totalphase.com/products/beagle-usb480/ . Saleae https://www.saleae.com also has some USB functions but not as extensive. It's much more versatile and look cooler though :)

None of these are cheap though.

The cheap option is https://github.com/ataradov/usb-sniffer which is actually by the same guy as the OP :)

Open hardware so you could build your own for fairly cheap. I bought a clone (is it a clone if the creator never sold them? I'd have preferred to buy it from him...) off aliexpress for ~$50.

If anyone has surplus bare PCBs for sale for the Cynthion please let me know.