Hello and welcome to the forum. It is not really possible for us to say what the best food is for your pug. All dogs are different, as are their owners and circumstances. There is a section on the website here
that explains much about what to feed, how much etc etc. It is a clear, concise article that is well worth looking at.
The amount of poo does seem excessive and it should be firm, not soft. Some useful articles on this here
You don't say which Burns product you were using but it seems a shame that you were doing well on the food but changed it due to the dog becoming picky. One way of overcoming this pickiness is to use a topper. This is becoming increasingly popular with users of kibble, probably because it adds taste and moisture. Burns do some wet foods, the one that comes to mind is Penlan Farm. All you would have to do is reduce the kibble slightly and top up with the wet food.
Alternatively (arguably a better option) would be for you to perhaps consider topping up with suitable home cooked food. Protein rich foods would balance the Burns quite well e.g.chicken, fish, sardines, minced beef. A small amount of suitable cooked vegetables and some fruit such as blueberries might also be acceptable to your pug. Check out our home cooking section for more information about this. The other thing that you can do to tempt the dog is to soak the kibble and add warm water immediately prior to serving. As we all know, scent is extremely important to dogs and this releases the aroma of the food, therefore making it more interesting to the dog.
Edit: Forgot to say that overfeeding can result in copious and soft stools so it might be worth checking that the amount you are giving is correct for your dog. It is useful to weigh food, using a digital scale in 1g divisions. Please could you post back to let us know the amount of Wainwright's that you are giving?