I have a couple of questions. Have you traveled in Europe before? If so, have you traveled independently? Have you taken a river cruise before?
We have taken one river cruise, the Viking Rhine cruise. We are normally independent travelers, but some long time friends asked us to join us on the VRC.
Here are some things you might want to consider. You are at the mercy of the river. From what I understand, things have gotten better, but there was a season a few years ago where lack of rainfall affected the navigation on the Rhine, Danube and others. For some, most of the trip turned into an expensive bus tour.
As river cruises become more popular, the rivers have gotten full of cruise boats. There's not enough moorings to service them all. We paid extra for a room with a small balcony, but more times than not we were tied to another VRC boat. Our view from our balcony was in the stateroom of the passengers on a sister ship. Our balcony was literally inches away from theirs.
Everything revolves around meals. The problem for us is we are early risers and usually start our day at least an hour before breakfast is served. That means you don't get to your first destination until mid-morning. You have a few hours, then you are hauled back to the ship for lunch. Then afternoon activities don't start until after 1:00, sometimes as late as 2:00. Then you have to be back to the ship for dinner, which normally begins serving at 6:00. Some nights you have to be back earlier because that actually start to the next destination while you are eating. There were a few days where we could opt to skip lunch and stay at a destination, but they weren't often. Carefully check the itinerary.
Luckily you are considering Avalon. Since they are a more premium cruise line, hopefully you'll have a better class of fellow passengers, though money doesn't necessarily keep people from being jerks. Most of the ~190 passengers were OK, but we had enough jerks on board to make for some unpleasant moments. The ships aren't big enough so you can avoid them. We had a couple of guys who couldn't hold their liquor and got loud and obnoxious. We had one guy who was just a jackass, abusive to staff and actually physically threatening another passenger.
It's an expensive way to travel. Circumstances had us spend the same amount of time in Tuscany on an independent trip 6 months before our cruise. The cruise cost us three times as much as our wonderful trip to Tuscany.
We had taken several trips before and have taken trips since. We quickly decided that cruising isn't our cup of tea. It's too constricting, it's too expensive and we had no interaction with folks in the areas we visited.