fwiw: Your post was a really big deal for me when I happened across it.
That board is what Sophos uses for their SG 105/115 UTM's.
I have three of these that have been laying around for a couple of years, and I've never been able to get around that error.
With my near total lack of BSD knowledge (and it appears BSD specific) I went a ways down the rabbit hole and got nowhere.
Toggled that usb setting per your post and the install was uneventful after that.
As a result, I now have three small pf routers that I can use for small projects instead of giving up my good hardware.
I'm getting around 800mbps as a pure router, with a load around 10%, so it's a nic limitation.
I should see what I can pull both aggregating and under a more real world load of packages.
In any event, it's an order of magnitude more performant than Sophos ever was.