I'll turn 78 in January and I'm also thinking about my next trip being a river cruise. I too am a solo traveler and it's a real challenge finding river cruises that give any breaks to solo travelers.
There was a previous thread where I whined about that and got this response, "LO - That's another reason we love Tauck -- there is NO SINGLE SUPPLEMENT. We have a single friend who travels with us - she pays the same thing we do. Now, you have to book the Cat. 1 rooms, but, trust me - the rooms are great. Tauck is also ALL INCLUSIVE. We've been on Gate 1, Viking, AMA, Avalon, Uniworld, and Tauck - Tauck is our favorite."
And this: "Lo, I have been exploring some of the links given above and also taking a deep dive into Gate 1 River cruises for a cabin with single occupancy and they do offer single cabins on the main deck in the back of the ship with a lookout window (no balcony or Juliet balcony). So, very basic but is possible to travel solo on their Danube river cruise with no additional charge for single.
Edit: my search link didn’t work but here is the main site: https://www.gate1travel.com"
And this: "Lo, I've read that on occasion Viking offers options for single travelers but you’d need to call them to find out where/when. There's a guy on Cruise Critic who frequently travels with his partner and mother-in-law."
This is the link to the rather recent discussion: https://community.ricksteves.com/travel-forum/transportation/river-cruise-companies-in-europe-choices
I'm not sure if or when I'll go, but I've been pleased to see that in spite of traveling a lot in Europe and living in Nürnberg 1982-85, there are places river cruises go and experiences they provide that would not be repeats for me.
As you do your research, please keep us updated on what you decide.