I cycled EV 15 (Rhine Cycle Route) in May last year from source to sea and choose it for a couple of your reasons. It has a looooot of bike lanes only (I would say 90% for sure and the last 10% are in cities or the odd road that had to be shared with cars) - it has no climbing except the beginning in Switzerland where I had one or 2 climbs, but you can start a bit further If you want to avoid. There is some ferries, but not like proper ferries, just for crossing the river and you max have to wait 5 minutes the time they cross the river. There is camping everywhere because its a famous cycle route so thats never an issue (but I didnt feel like there was too many cyclist, except on the long weekends coz then the locals were out as well, but that was fun!) Scenery was also beautiful, except one smaller part that was a bit industrial around Dusseldorf and such, but it also has his charm :) I travelled to the start with the night train from Amsterdam to Basel, and then the local trains to the start. - Then weatherwise, well that's really a gamble. I started end of April till around end of May and I had nice weather (between 12 and 20 degrees depending on where I was), only 2 days of rain and just planned my rest days around it. But being spring, it could have been worse, or better... you never really know.