Thank you for the kind words! So it actually does support true billboarding (100% camera facing). Try this! 😉
DrawSprite3D(sprite_index, image_index, x, y, z, -CAMERA_PITCH, 0, -CAMERA_ANGLE, image_xscale, image_yscale, image_zscale, image_blend, image_alpha);
You just have to account for the 'camera pitch' when rotating the sprite! Be aware that you can potentially get clipping issues since if you imagine not looking from the camera's perspective you are rotating the sprite around the xyz axis. if 'z=0' then once you look over the top he will more than likely disappear into the ground. :P