I use ViewRanger with an 820.
• Plug Garmin in to Mac to charge it. Meanwhile…
iPad:
• Open ViewRanger, create new route with pencil.
• Export route to GPX; sync with DropBox.
Back on the Mac:
• Copy the new GPX from DropBox to ‘New Files’ folder on Garmin.
• Unplug Garmin, put it in ‘GPX’ mode*, select Nav>Courses, find the one you’ve just uploaded.
*GPX mode: https://ridewithgps.com/help/garmin-edge-820
EDIT: I don’t know if this will work with an Etrex or not. Must be similar, though?
And, just saw your note about using the phone. I used ViewRanger to navigate on the bike for a while; I soon got bored of stopping to take it out of pocket, refresh the view, etc. The Garmin has the advantage of being small and always there on the bars. But I suppose you could do that with the phone too.