If there are flats and ponds, I suggest you build an ecological chain farm.
Plant cash crops and some plant crops that livestock can eat in the mountains, and then raise pigs to find their own food once a day (someone needs to look after them, or they will lose money if they run away). Fish will eat the excrement of ducks and fish in fish ponds.
Specifically, look for information about ecological chain farms. This method can mainly save the cost of raising crops and fertilizers.