I live near both the Mosel and Rhine bike routes, and visit along both rivers with regularity. There are major advantages for considering either of the routes between Trier and Koblenz (Mosel) or Mainz and Koblenz (Rhine). (And yes, you may want to start/end elsewhere, but stay between those points.)
Both of these paths are wide, well paved, and run along the rivers (with some minor deviations) separate from the automotive roads. As such they are fairly flat. Also you have hotels, parks, places to shop and eat, all along the routes. you also have the trains, where you can take the bikes, and that allows you to base in one place and ride out from, or back to, that point without having to bike both directions (unless you want to.) And both of these routes also have bike rental businesses for both conventional and e bikes. It's extremely common to find families biking these routes on holiday.
I prefer the Mosel, but you'll get a lot of good input on this forum regarding either. Note that the large majority of the towns on either route are small, with limited attractions other than the food, hospitality, and scenery. That being said, the major cities (Trier, Koblenz, and Mainz) all have much more in the way of museums, historical points of interest, etc. Still, either f those routes will offer, castles, river cruises, train rides, hiking, biking, etc.
I can also comment on the trails around Lake Constance, as I visit there annually. This is rather different, as much of the bike path will be on roads shared with motor vehicles. These small towns, and the few bigger cities (Konstanz, Lindau, etc) have some bike routes but many of the small towns do not have dedicated lanes. And the roads do not stay right along the lakeside. Plus you don't have the same level of transit that allows you to move your bikes when you are not riding, although there is some. Still, there's a lot to see and do around the Bodensee.
I really don't see the need for a planned tour. The bikeline books are excellent, with detailed maps, and when combined with some of the apps available and a little research, it should be easy to put together a week, or longer, tour.