Most of us who have or will go in for facial surgery have Type III foreheads. Do not be concerned about that. With the Type III surgery, they'll cut out the brow ridge and replace it with a prosthetic made either from exogenous materials or with pieces of the just-removed bone.
With a large brow ridge, implants will not change the gendering you receive. It isn't just the shape of the forehead, but how much it protrudes over the eyes. Implants can round out the forehead, but won't change how much the forehead itself sticks out.