Just to add two cents, the route planner over at komoot.com (not a link, I'm new) seems to make sense if you're exploring options. For additional data, there's also heat maps from Strava and other offerings (Movescount, ...). If you select "Bike Touring" in komoot, it will give preference to cycleways over roads, and will indicate distribution of way types and surfaces as you tweak the route. You can't beat local know-how, but this could be a supplement.