Best way to attach Shelf Brackets on a Drywall for hanging Light Fixture without using Stud?
That's a quick drawing of a shelf bracket set against a wall.

This is basically a bent lever, so if the weight of your light exerts a force F1 on the end of the bracket, which extends a distance A from the wall, then the bracket will pull on the anchor with a force F2 = F1 * A/B
TL/DR: pick a bracket with a long vertical part (B) to minimize the pulling force applied on the anchor.
Since both your brackets have B shorter than A, the pulling force on the anchor will be higher than the weight of the light fixture. Therefore, you might as well stick the anchor in the drywall ceiling instead, and forget about the shelf bracket. In this case the anchor will only bear the weight of the fixture.