I'd concur with your assessment on size versus road surfaces. We didn't exactly travel light, but enough to use 700 x 28s on our RTW trip, mainly on bitumen. The tyres were Schwalbe Durano slicks.

The 35s I've swapped to on the touring bike have been great for the gravel rail-trail and dirt roads in our area since we go back. Schwalbe Marathons.

And I have done a fair bit of heavy-duty gravel road touring in the past, but on mountain bikes with up to 45mm, but in 26" version.

Much does depend on comfort levels for the conditions and desire ease of pedaling.

On that latter bit, speed does become irrelevant in terms of rolling resistance. But certainly, the cumulative effort required to pedal 60 or 70 miles does become important. I'd rather ride an easy-rolling tyre that reduces my effort on the flats and up hills.