Have you tried building your own firmware? It's not so difficult: https://openwrt.org/docs/guide-user/additional-software/beginners-build-guide
When you have set up the build environment it is trivial to turn off USB3. It's under Kernel modules / USB Support in the make menuconfig menu.
I don't know of a way to switch it off in realtime if rmmod doesn't work. My guess is that they have added the functionality themselves.