I cycled the eurovelo 6 last year. I used a specific mapset by Huber these were pretty good and a series of 6 booklet maps which folded out like a snapfax showing the route and about 10km either side of the route. I believe they do maps for most eurovelo routes. On the garmin was downloaded OSM and all campsites loaded as POI downloaded from Archies camping.
However the route was so well signposted, the maps were never used for actual navigation but for rough planning as to where to stop for lunch / afternoon wine the following day… When I do another Eurovelo I’ll quite likely just print off the overview maps.
The archies camping POI data was a godsend though – we never booked ahead but at our 3pm wine stop we’d decide if we wanted to stop at the municipal campsite in 5,10 or 15km then use the garmin to navigate to it.