Sounds like you're planning on 4-5-hr. minimum driving days for A'dam > Rhine > Nuremberg. Normally, Boppard is my go-to suggestion too, but you are staying only one night, and with so little time for the area, it does not sound like you are planning to do a Rhine cruise, or tour Marksburg Castle, or travel up the Mosel to Cochem or Burg Eltz, or wander around Bacharach, or do any hiking, all the sorts of activities that make Boppard such a great base town. Your only criteria seem to be "small" and "picturesque," which means other towns might work just as well in your case.
The other factor is your timing. The Middle Rhine Valley is having one of its biggest weekends from Sep 12-16, the Oberwesel Wine Market festival, and ALL the left-bank Rhine towns will be heavily impacted, with crowds, traffic, and overbooked hotels the entire time. Saturday will be the biggest day of all. In Oberwesel, Bacharach, St Goar, Bingen, and Boppard, whatever your hotel criteria might be, you might be lucky to find anything at all, much less what you are looking for in a hotel (which I assume is a room with a nice view, maybe air conditioning, at a minimum.)
So you will need to keep your option wide open, I think. Perhaps the right-bank towns will be a little less busy. I have stayed in tiny Osterspai previously - a quite, lovely quiet town just upstream from Braubach and just across the river and north of Boppard.
https://www.youtube.com/watch?v=V4E5FyzrK98
https://osterspai.welterbe-mittelrheintal.de/urlaub-und-uebernachten/gastgeber
Braubach itself, just a few minutes from Koblenz, is also small, with a lovely old town zone and a few places to find dinner. Scroll through these 3 pages of photos to get an idea:
https://www.stadtbild-deutschland.org/forum/index.php?thread/7322-braubach-am-rhein-und-marksburg-galerie/
https://www.tripadvisor.com/Hotels-g642094-Braubach_Rhineland_Palatinate-Hotels.html
Or perhaps you will find something that suits you on the nearby Mosel River, or in the Lahn or Nahe River Valleys.