Thinking a thought is like watering a plant in a garden. Your attention is the sprinkler. The more you water a plant (up to a point, of course), the more the plant grows.
Similarly, the more you think about a thought, the more that thought network grows. The denser a thought network, the likelier it is that you will end up thinking about/through that thought network. There are more entry points and the paths are better paved.
In other words, thinking thoughts make it likelier that you will think those thoughts in the future. This can cause psychological rigidity.
However, psycholofical flexibility can be developed through mindfulness. In particular, I am talking about mindfulness developed through meditations like mindful breathing. In that kind of meditation, you start by noticing your breath. When you're distracted by something, you pay attention to it, but you return to the breathing. The point is to develop flexible attention. You choose what to pay attention to, even when your attention is pulled by something.
That is why I say that experienced meditators would notice earworms just like anyone else (after listening to the song or remembering it because of another related memory), but because they can choose not to pay attention to it and feed that thought network, there is a lower probability of having those networks reinforced. Their sprinklers can turn off with more ease than non-meditators'.
Meditators can choose not to feed the cognitive network. Non-meditators could find themselves feeding the network.
Thanks for sharing your method.
As to your take on Anki, I think it's fair and accurate. I agree with you in that the learning curve is not in the features or the interface, but as you said: in the pacing. I really hope I can try to space the cards as much as possible, so that a regular practice doesn't become burdensome.