Two things:
Sadly, my USB port completely borked, so that's off the table. I use KDE-something-or-other to txfr files over my internetless wifi.
Second, I did look into it, and even on my Graphene OS phone, the USB tether is treated identically to the hot spot; that is to say, the carrier can detect you're connecting a device and there is no workaround other than paying the cellular provider for the privilege of using your connection as they see fit.