It's certainly not less effective with higher concentrations. Some dermatologists say that 5% niacinamide has the same results as 10% but it's less irritating. On the other hand, some claim that 10% niacinamide is better for acne, inflammation and redness.
Niacinamide is usually well tolerated even in higher concentrations. So if your skin isn't sensitive, there shouldn't be any issues even with a 10% niacinamide serum. If you want to go lower, some brightening serums have ~4% niacinamide, some toners too, but it's usually not in the name of the product, so you have to search by ingredients.