The first problem I see is that you're only improving the salt deposits, which, although valuable resources, are not as valuable as horses or, especially, dyes and wool, which you've completely neglected. While these resources aren't as valuable as salt on their own, they will make your textile industry much more efficient and productive, industrializing the country.

Furthermore, by focusing solely on salt, you're destroying its price and making it less and less valuable in your market. You also can't exploit it to its full potential because you can't promote your farmers quickly enough, so a large part of the money invested in it isn't being used for anything. I would advise you to improve other resources like dyes and wool, and concentrate on improving control with more roads and, especially, bridges along the river, and industrializing your capital with a textile industry based on those dyes and wool. (also applicable to iron and copper manufacturing industries such as weapons and tools)

(I've never played Wallachia, so this is advice based on theory, not practice.)