Molyneux's most sustained effort at systematic philosophy is the book Universally Preferable Behavior (2007).
Upon publication, it was the object of a memorable take-down by David Gordon, a senior fellow at the Ludwig von Mises Institute.
Here is that review: https://mises.org/library/molyneux-problem
One incidental fact: Gordon's review is headlined "The Molyneux Problem," which is a sidelong reference to another Molyneux from a long time ago.
It was in 1688 that an Irishman, William Molyneux, wrote to the English philosopher John Locke asking him about an issue in the psychology of perception. He asked whether a man who has been born blind, and who has learned to distinguish between a sphere and a cube by touch, would be able to distinguish the sphere from the cube if he did gain sight in the course of his adult life, by sight alone. This is the original Molyneux Problem.
Why is the historical pun relevant to the book? Gordon's view of Molyneux is that of someone who learned of certain philosophical issues late in life, and who quickly convinced himself he had solved them. Perhaps the (formerly) blind man sees shapes for the first time and wrongly believes he knows which one is which and, blissfully unaware of his own ignorance, doesn't bother to check this ought through tactile examination.
Gordon's review (combined with my own slight exposure to Molyneux's superficial philosophizing in an audio of a lecture of his to which I listened a couple of years back) has been for me sufficiently persuasive that I've never moved on to the book.