Is Chartres going to get any M routes?
Nope. It is not a
Métropole - no M routes.
I notice D routes with the same number on either side of the city (and in at least one case, OSM signs it through).
It's difficult to find the best way because some towns generally don't sign D routes within their boundaries but some do. Many towns have old signs (especially N route signs) still in place decades after they were downgraded or albeit bypasses are in service now. It is always an individual decision for each town whether I draft routes through towns, up to the town boundaries (like in Chartres) or up to a town bypass but I try to be consistent for all routes of the town although there might be some signs within the town for individual routes.
Chartres seems to have signs up to the town boundary only. However, feel free to prove me wrong with GSV links