What is the optimal configuration of a cross brace on a wooden gate?
You asked for optimal: Follow a few thousand years of practical experience and put in a tension brace (lower outside corner to top hinge-side corner - opposite what you are going for, which is a compression brace) Go with the past few hundred years and make it a turnbuckle.

The best form of compression brace "in plane" is none of the above, and has a point on the end that connects to both sides of each corner, with the midpoint of the brace hitting each corner. Call it corner to corner. That is inferior to an "out of plane" brace in the same position that overlaps the backside of the face frame and is glued and screwed to the face boards; both are inferior to the tension brace, though you can use both for the classic X brace scheme - but the tension side will do most of the work.