I'm watching a YT channel of a couple riding one of the Euro Velo Routes, this one follows the Loire Valley, from central France, then north and west to the Atlantic. Almost completely flat and on off road trails. Its beautiful, the towns look wonderful, great food and wine. They had done a different tour a few years back Velo Route 6 from near Zurich to Vienna, but you can grab it on the Atlantic coast and follow to the Black Sea. It mostly follows the Danube for a while, is also flat and mostly on trails.
Look up the various Euro Velo routes, they are all over, very well signed and easy to follow. As well you can catch a train easily between start and finish points, so logistics are easy.