As the introvert half of a couple with a very social partner, I prefer large tables to 4 tops on cruises because I can be silent and he can interact as much as he wants. That's basically how I handled our Avalon european river cruise. Though that was different as 2 sets of parents traveled with us. Our Mekong Avalon cruise was an even smaller ship but for some reason it didn't bother me there - there were 3-4 other groups/people on the trip I interacted with regularly and the rest I just...didn't. It is entirely possible to be an introvert and enjoy river cruising. (esp as there were only 5 of us who regularly sat outside on the ship along the Mekong, who didn't mind the weather)

Look at photos of the river cruise ships' dining rooms - banquette seating often allows small separations so you are able to sit as "just a couple". Close enough to interact if desired, but you also can avoid eye contact if you don't want to speak to your neighbors. And these days, there are a few river cruises that offer room service, if that is super important to you.