Basically, as it is right now, the lense flare is either visible or not, that's not quite correct as the scale should reduce as the source gets partially obstructed.
Use diffraction spike mode "simple", this will cause the gradual dimming of the flare as it is obstructed. Note that this only works when a perfectly round world eclipses a sun as seen from space. It does not work with landscape features. If you are close enough to the star you can use mode "super" to provide a more realistic appearance with the same benefits as "normal" mode, but this will cause a large performance impact.