Your testing seems pretty thorough to me. If the performance is good from USB3 to a wired computer on the non-bridge side, and the 5ghz performance is good between computers on each side of the bridge, then it points to a problem specific to the media bridge/USB drive combination.
If you can try another USB drive on the same port (the test usb device doesn't have to be that fast either, just faster than your current USB-to-bridge transfer speeds), I would try that just to rule out the drive itself.
If it isn't the drive, it seems to me that it's pretty likely to be an issue with the router's firmware. If so, it could be fixed with a future update. Asus was changing the USB functionality to support USB hubs in the past few versions and it's possible they introduced your issue then. You could try a firmware from before the USB changes (I think those first appeared in January) and see if USB over media bridge works better (but I wouldn't stay on those for long, the recent ones have better performance and security).
If you can try another USB drive on the same port (the test usb device doesn't have to be that fast either, just faster than your current USB-to-bridge transfer speeds), I would try that just to rule out the drive itself.
If it isn't the drive, it seems to me that it's pretty likely to be an issue with the router's firmware. If so, it could be fixed with a future update. Asus was changing the USB functionality to support USB hubs in the past few versions and it's possible they introduced your issue then. You could try a firmware from before the USB changes (I think those first appeared in January) and see if USB over media bridge works better (but I wouldn't stay on those for long, the recent ones have better performance and security).