Holy thread resurrection!
Today, I noticed for the first time, that Wahoo have done away with the need to "ping" (their expression) the cloud to process a new route.
They said they would look into it and (a few years later!) they have!
This means that it is now possible to create a route totally offline (Osmand) and transfer that route to a Wahoo gps device with absolutely zero need for data or internet connectivity or cables/adapters.
Can't comment on Garmins, but this is a great boon for tourers in remote places.