Aren't income taxes a logical fit here? You would still have graduated tax brackets, so you wouldn't have an enormous cliff once you started earning, but all tax brackets could be increased such that the middle class end up about the same, and the poor are better off. With the added efficiencies described by the parent comment, this should even be possible without reducing net income to those in the higher brackets, but it might be desirable to do that to some extent as well for greater redistribution.