Really nice spin on the theme! Interpreting trade routes as loops never occured to me. I really like the aesthetic of your game and the concept is cleanly implemented. Would have loved to see the rules more clearly stated, like for example that a ship can only load 1 item of 1 good so one supply harbor can only satisfy 1 request of its good downstream in one traderun. But I guess the mini metro inspiration implied this was the case. Good job!
I also encountered a bug where adding a trade route with only 1 harbor crashes the map ui.