We did the Douro last Fall with Tauck and we found it the sleepiest of all the river cruises we’ve taken. Tauck is known for having the best excursions and guides and even with that it felt like a stretch in some of the ports to make the stop interesting. I’ve done the same rivers as you and really loved the Moselle. The Douro was recommended to us as being the most like the Moselle. They are similar but the Moselle has much more interesting towns. Of course on the Douro there is history, cave paintings, ruins and of course wine but the big draw is the beautiful scenery. It is a really picturesque area and because ships cannot sail at night you get to see a lot of it and there will be quite a bit of down time. The crews are mostly, if not all, Portuguese and really friendly and kind. It should be enjoyable but this was the first cruise we felt ready to end it by the 3rd day. Having had the experience now I would recommend just DYI and drive the area and perhaps just take a day boat along the river as an excursion.