This is a brief overview of the first half of the second section of Kant's Groundwork of the Metaphysics of Morals. For students, I recommend The Annotated Kant: Groundwork for the Metaphysics of Morals by Steven M. Cahn et al.
After days of searching, I came across this. You summarized the points with accuracy and sufficient reason, following the sequence of the Groundwork. All other summaries seem to make Kant sound too arbitrary and impractical.