I've gone for a GRX double on mine, as the little ring (31) is only slightly bigger than the smallest on a triple (usually 28). With touring in particular though, I think it's all about having *your* bike, rather than "the best", especially when it goes anywhere and everywhere with you.

Really hope that they will carry bikes, as they do for Brussels - Prague in Summer.

The Rhine section from Mainz to Köln (which looks like it's on your route) is brilliant!