We typically travel mid October to early November. Crowds are low and temps can range from cool to delightful. Agree with comments that castles on the Rhine are best viewed rom the top or front deck (with a cup of coffee or tea). To me, this itinerary was a slower pace than a Danube cruise (which sometimes had one stop in the morning, lunch on board, and another stop in the afternoon. We didn’t travel with Viking, so I can’t comment on their excursions.
if you’ve never been to Amsterdam before, go a few days early if possible. There is so much to see! Been there twice and still would love to return again.
as others said, there are a lot of other river cruise lines to look into. This forum has wonderful helpers so don’t be afraid to ask!.
also you say you are seasoned cruisers, river cruising is VERY different. But to me different in the best ways. Not thousands of people, not multiple restaurants, no casino, no shows. But wonderful people (both staff and guests), excellent conversation, beautiful locations, amazing experiences.