Would "middle of the game" be synonymous with "Mexico"? Mexico on its own, as landscape, was really nice. Landon Ricketts' missions were good fun, and felt perfect for the world. The rebeldes' and generale's missions were not my cup of tea.
You should download the DLC for trading and hunting, which is free. I think it conveys a cash bonus for trading. Also, you should try staying honorable, as it positively affects prices everywhere except Thieves' Landing.