NOTE: This article is a work in progress. Apologies if it is not quite polished.
I have used the Garmin Etrex Vista HCX. Etrex Vista Review. I bought it for the Ride Earth trip. I put it on to record a track log of the route. I didn’t use it for navigation until the Mongolia trip. Although now it looks a bit old school, Garmin still does this kind of product.
Garmin has an app called Basecamp for route creation. How to open maps in Garmin Basecamp.
You can get maps to import from:
Free from Open Street map
I pay for the OS Maps subscription. I regularly use the app on desktop to create routes.
On Android I have used the Guru maps app for open street map maps
Other interesting stuff
GPSBabel converts waypoints, tracks, and routes between popular GPS receivers such as Garmin or Magellan and mapping programs like Google Earth or Basecamp.