My 2 cents.
Thoughts are a form of fabrication, so to try and find out the non-self of them by thinking it out is like trying to fight fire with fire. It only makes it worse.
Of course contemplation is not bad, even useful at times, but insights are to be found behind the conceptual level of thoughts. So one of the things in meditation should be to quiet the mind down. Then one can start to see the "fall" of thoughts and from that experience start to see their non-self. Knowing this more clearly, the mind automatically becomes more silent. So then thoughts can be gone for even longer periods of time.
The same with the other aggregates. You can't concentrate and suddenly decide: Now I'm going to experience the falling away of form, the senses, vedana etc. This happens BECAUSE of the deepened concentration. Then afterward can you reflect on them passing away, not before. So I'm with Ajahn Chah on this one: Satipattana and anapanasati work in unison and aren't really separate things, they are the same.
It's not like one day you are the worst meditatior in the world and the next day you know everything and are totally calm. Apart from the stages of enlightenment, it is mostly a gradual path with insight and calm supporting each other.