Disagree. TGV going to Frankfurt, but get off at Kaiserslautern and
change to train to Bingen,
As far as I can see on Bahn.com, the majority of TGV going to Frankfurt pass through Strasbourg, on essentially the route that I describe. You can go through Brussels, but that's kind of a milk run. You can take the TGV to Metz and then four more trains north up through that relatively low population area to get to Bingen. There's one TGV INOUI a day in the summer that bypasses Strasbourg, 4 hours to Bingen.
And then get on a boat I suppose and head northwest to Bacharach or thereabouts. From where you are timewise now farther away from Venice than you were when you started in Paris.
Or you could blast straight to Basel on the TGV, and then start heading up the Rhine going east exploring all of the interesting towns in that interesting area. Travel toward Venice, not away from Venice. Granted that area draws more German tourists than American, but I do think it holds up to the Rhine Gorge as a place to visit quite well. In my estimation anyway. Without denigrating the Rhine Gorge, in my opinion after having travelled the entire Rhine river (and most famous parts more than once), the Gorge does not stand above some other parts of the Rhine for beauty or interest, nor many parts of many other rivers.
If one must have the Rhine Gorge, I suppose it is worthwhile to go the opposite direction of where one wants to end up to see it.
But anyway my suggested routing is just that, a suggestion. There's nothing particularly unpleasant about town hopping the Rhine Gorge, then seeing Bern Switzerland, then taking a train from there to Venice. I'm sure that would be fun enough :)