Just a note of caution: the gradient needs more trial and error. Today I tried to push the gradient even steeper and it did not work at all. Only half the design engraved.
I think the approach to tackle uneven brightness will vary from design to design. If the design elements can be “layered” with different color fills then you may not need a gradient. Each color can be engraved at an increasing power setting (shallow to deeper engraving). Just arrange the elements such that the deepest engraved design element is the furthest from the base.
Also the uneven brightness seems to affect the bottom center portion of the design most, about 2 inches from the base. Any design element at that height should be the shallowest engrave (low power + high speed).