GPS tech (Garmin, Hammerhead, Wahoo, RWGPS, etc)

Would I be able to load RWGPS routes into that? I guess that's just a GPX file?

The etrex can take both gpx tracks and gpx routes depending on which model.

You are best with gpx tracks if using very old models such as the etrex H or vista due to the limitation of 50 waypoints on a gpx route.
There is a limit of gpx track points but this is 10,000 track points.
Most route!! creation websites actually create gpx track files not gpx route files.