ok so this might work, and is a hack
when rndis is enabled, usbethx is automatically stopped.
so set
setprop persist.sys.usb.config rndis,adb
and then connect the ethernet
im not sure if this will work or not, im not near hw atm. also im not sure on timing
So… it worked, but it also disabled adb 
. So I’m locked out, trying to find a way back in (I did not think about installing a terminal emulator before trying that 
).
EDIT: it got back quickly and I could change it. I wouldn’t advise anybody to do that (unless I missed something). However, a variant may be to go to Developer Options > Select USB Configuration > RNDIS (USB Ethernet). Which changes sys.usb.config instead of persist.sys.usb.config.
EDIT2: actually now usbethx never comes back, even after a reboot, even if I run start usbethx. Not sure why
adb would have been there, but you do get booted as soon as you run that command
and yes this persists.