Yes a cam would allow fine changes with just one control arm. The mount would have to be spring loaded to the cam to hold it tight, but a long control arm on the cam could compensate for any high-drag that the cam would put on. Probably a more positive means of adjustment than the other two approaches, due to less slop. Coarse adjustment could be made with spacers at the transom. Thrust points would force the cam to be located at the least optimum location at the bottom of the mount, and hinge point at the top - which exaggerates thrust angle changes. Aside from machining the cam itself, probably the simplest most tidy approach mechanically.