This has proven to be quite tricky but we have learned an enormous amount in the process.
Fortunately there are quite a few tools for this job as well as an enormous amount of information available on the internet - the trick is just to find it and make sense of it.
The websites I used that proved to be enormously useful were:
www.komoot.com was the main site that I have used to design the route. There is a wealth of information available available here. Due to the length of the route, I had to break it up into stages and work on each stage almost as if it was a separate ride.
I used information from many websites as well as facebook groups to decide on which roads/ paths to take and finalized all of this in komoot.
Once I had all my separate routes defined, I exported the information to GPX files. These files I then merged together using www.joewein.net/bike/gpxmerge/
Once completed, I uploaded this one huge GPX file (about 22Mb) into www.ridewithgps.com. This gave me the complete overview of the entire route.
I will use these komoot offline files as well as the GPX files downloaded to my Garmin for my daily navigation.
The value of Wikipedia, EuroVelo ( www.eurovelo.org) as well as many other sites on specific towns and districts proved to be enormous.
Lastly, many people have wonderful experience gained from riding in Europe, and they have been kind enough to share this information freely with my on Facebook. To them I tip my hat! Thank you