Eyes are pretty jittery in general, but try upping Eye Gaze > Smoothness to something like 100% and see how that feels. Smoothness will basically clean up the rough edges and ensure every transition is more gradual.
I've been putting your suggestion to the test. Working just with the Eye Gaze Behavior.
The Smoothing and Camera Strength certainly have an impact on the pupils behavior, but I can not get to a satisfactory settings, between both, that settles the eyes. Either the jittering is cut to a minimum and the eyes move around as if a slow, drunken state, or lots of jittering without hardly any lateral movement of the eye!?
It seems as if the setting, both Smoothing and Camera Strength, work at the extremes, with little impact in the middle of the settings ie, Smoothing works best between 0-10, or 90-100. Could these settings be made more linear, so having an impact throughout the range?
I take it the movement of the Eye/Pupil, is managed via the Pupil Tag, but if my eye is steady, what makes the eye jitter?
I'm not sure I agree that, 'eyes are pretty jittery in general,' that is unless you have a good/bad night, depending on your perspective!
Any jitter is probably a combo of your actual eye movements + the face tracking readjusting if it can't find the pupils. The closer and better lit your eyes can be in front of the camera, the better the results should be. In general I feel like I don't run into this issue much, but I also usually do mouse controlled pupils for any recorded work to get more precise control. I agree that the smoothness range in particular can feel minimal unless you're dealing in the 80-100% range - we could do a better job with that.