Are bike shorts a good idea?

Yes, wear bike shorts. I don't know which trip you are taking, but I took the Alps to Verona trip in 2015 and wore rain pants over my shorts only on the first day when in rained in early September. It was quite warm as we went south. The bike saddle was moderately comfortable, but the smart money brought padded seat covers with them. In 2016 I did the bike and barge trip in Holland/Belgium and it was the most comfortable bike I have ever ridden. I did wear padded bike shorts. It became unseasonably hot last year and no one wore long pants. Take knee socks if you think it will get cold or biker capris. When I ride in the winter i wear woolen knee socks, like ski socks.