Brabanters are supposed to have brown tails with black dot on the tips, right? I just browsed pictures and it is a little confusing on the golds but the silver ones make this very clear(as in white tail with black tips).
Seems a little detail but it makes a huge difference because that shows they are based on ER/birchen. On other bases- eb/partridge, etc the tail will be solid black.
Spangling of the black dot at tip of feather requires Pg, Ml and Db, on Birchen for the spangled tail.
Birchen NN probably would be the best outcross..
Nearly all hatchery stock NN are wheaten plus Co to make them black tail buff/reds. you don't really want wheaten or Co, because wheaten can make the spangles pretty small or have poor spangling on breasts- it "likes" to get rid of black pigments, which is why it is so often used for buff birds. Adding Co to the mix can give some birds that visually look spangled but it's fake.. but it also will make a whole series of patterning with the Pg Ml- pencilling, double lacing, even single lace etc. They can create very beautiful or cool patterns but can be a headache if trying for a single pattern.. it is do able but you need to know what the clues are so you can pick out the correct birds for the next step.
Co is "invisible" in blacks and birchens, so the black/birchen hatchery NN very likely have it due to it being so widespread.
Not end of the world though.... especially if you are willing to breed back the crosses back to pure brabanters, that will make it so much simpler and there will be more examples of the correct genetic combations for you to pick out in the second generation onwards.
