You want to set Diplomacy Economic and AI changes to medium or high, at Phantasy Settings in the camp menu. But it will never hit 10K like you wished. At high/high I had 2.2 K showing at the dwarven capital, which declared itself "prosperous", after buying some goods the previous week.

The change happens NEXT payday, not instantly, and roughly every 4 days after that when the merchants restock each 95 hours (instead of "standard" 1 week).

Of course there are other ways to get cash quickly - you could chase epidemics with a fairly large party and some talented healers, for easy money in the thousands per day (at a town). The sum is based on the center type (less for castles, less still for villages) and the raw size of your party, without caring as to their level or tier. It helps to have good party healing in all three healing categories, as these get summed to determine your effectiveness.

You can also try moving around to offload items at more places, especially iron or salt you picked up cheaply at either dwarves or orcs, depending where you prefer to spend time. A ship can help move quickly.

I don't really feel it is set wrong, standard diplomacy caps the merchants and you already have plenty of income possibilities, but you DO need to move around.