I think it would be impossible to get much if any straightening of that trunk - better to look at styling options around the existing trunk shape. Sorry if that seems a bit negative, but it would be a shame to risk ruining a well developed tree by trying to radically reshape the trunk. If you are really keen to lose the lower sharp S bend it is more practical to consider an air layer above that point, ie to grow a new root system higher up the trunk and separate the top part once that is viable - I have one in course at present but I don't expect to be able to be able to remove it for a few months yet.
Dave