It appears to me the straight section was retained as a sacrifice branch to thicken the base. I would start by removing it and replanting on a better planting angle. Grow out for several years to heal the scar and develop the trunk. Toyo Nishiki thicken slowly so it will take some time.
For best results be careful when removing the sacrifice trunk and seal carefully for best results. The line is not intended to be the finish cut just to suggest where to start. The actual cut should be planned to give a natural transition when healed. I would cut just before growing season in your location.
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