Sure some of the once you list are people who travel. The Herdsmen, sure but there are only a select few places where people live that way, also most of the areas they travel are underpopulated by nature and probably they sell there exes livestock at the same market every time.
The artists, well most of them would look for a benefactor that they can get paid by to produce art. Sure they might travel but I think most of them would stay in a big city looking for work. As traveling takes long time, that is time they don´t work or get money.
The construction workshops and people, well most buildings take a really long time like cathedrals so they can employ people for literally generations. So many of them would not travel but learn there craft on site, live work and die. And houses and such do not employ the big number of these people.
Farmers, well I am not really sure here. Most of Europe have a feudal system in place where the farmers are as close to slaves as can be without being actual slaves. Also the market they would travel to would be a market that would be as close as possible. If they travel to a market at all.
Almost forgot the sailors, most of them are probably fishermen. They just fish locally of the coast. So very few would do any actual traveling.
People going to towns looking for work. Well not really sure what type of work they are looking for. Really before any industries pop up there would be very little work to be had in cities. Most professions have apprentices and they more then often are paid positions.
I would say the most traveling the common man did was probably during wartime. Either drafted into a army or fleeing a enemy army approaching. Also there is the crusades, they had a ton of people travel a very long distance. But That would at best be a certain type of traveling. This type I would argue do not expose the people to any local customs or languages like say a tourist would be.
Also the clergy, they probably have a sort of good % of people traveling.
Just saying that sure people did travel. Just that it is way way more rare then you normally would think. Also I would argue that proper travel is not a farmer going to a local market. But more a merchant traveling to a neighboring region or country.
Stadtluft macht frei: I did not really see anything hinting that people where moving to towns to work. But rather to escape the feudal system. Sort of the slaves escaping north in the US. But sure, many people in the same places creates opportunity and work. But I would not really compare this to the scale of things during the industrialization.