This is all what i want to say.
楼上两位意思都很正确,不过语法有问题:all后面不可以跟what, 偶觉得是Those are all (that) I want to say. ---书面语加that,口语不用加
This is all I wish to say.
This is what I want to say. 三楼说的反而是画蛇添足,即是在书面语种that也是可以省略的,而且不需要将that改为those