I think this is generally right, most of the routes are set up as loops. So, you tend to finish right at the start of the loop. To the OP, don’t forget that each route has a lead in, which is usually pretty short. If you keep riding on the same route, you won’t cover the lead in distance again. This shouldn’t matter most of the time, but just a heads up.
Another very minor point is that for routes going up Ventoux and the Alpe du Zwift, there’s a roundabout at the top of the mountain and you go back down. Once you’ve descended, if you want to climb again in the same session, you will need to manually U-turn. What kind of person would want to climb those mountains multiple times?
The Keith Hill After Party is interesting. It looks like if you keep riding past the finish, you should eventually encounter a decision point where the game could either route you back to the start in London, or make you keep climbing Keith Hill. I am going to guess the former, but maybe I ride the Keith Hill After Party route during the Tour de Zwift and find out.