There are no potholes in Bavaria. Potholes are not a known concept here. I entered Bavaria via Lake Constance and exited today via Füssen, didn't see a single pothole the entire time. Same story in Baden-Württemberg where I was before Bavaria. Forget potholes, there are hardly even cracks on the roads here. Even single lane farm roads in southern Germany are smoother than the road in front of the freaking White House. I don't know what else to say other than that the German government clearly has a much higher road budget than America. But we all already knew that right? They are famous for their country's immaculate driving experience.
I'm fully on board with the idea that wider tires will indeed smooth out bumps. But what bumps? The wide tire has nothing to smooth if there are literally zero bumps in existence.
Adapt you tire width choice to the conditions of your chosen touring route, that's all I'm saying. If you're touring in an area that has bumpy roads, wide tires are a great idea. And don't forget to consider the entire route. I could tour on 28s in Germany, but 28s wouldn't be wide enough when I get to the Balkans next month, so I'm using 35s even if they are pointless in Germany, because I don't want to change my tires every two weeks.
The Germany example is a particularly exceptional one, I've noticed the same in Germany and in Europe generally, even the UK, the road surfaces are tons better than our roads here in my part of Canada.
As you say though, it's all about what you can expect and figuring out a good compromise -- and if you think you will be on gravel roads or trails or whatever sometimes, having wider in the range of 38-40mm is a good compromise for comfort and float with lower pressures , but a very little speed difference compared to a 32 or 35.
I've just put on some 38-40mm slicks on my touring bike and they are particularly light and well rolling tires, and compared to riding my commuter with regular Joe Blow Marathons 35mm, there's really no difference, but 40 compared to 35 does have a nicer ride over the real life paved road conditions I ride all the time.