Do you recommend looking into any specific more complicated theory or guidance into meditation, or do you think it's sufficient to focus on breathing while letting thoughts flow and be?
One excellent text that comes from one of the parent lineages of Shambhala is the Tibetan Book of the Dead, which is basically a preparation for the Bardo, or the "in-between" stages after life. You can read more here: https://en.wikipedia.org/wiki/Bardo_Thodol
There is a vast library of teachings from our senior teachers here: http://shambhala.org/teachings-library/ Plus a bunch of recorded talks by our very controversial founder: https://www.youtube.com/results?search_query=chogyam+trungpa
For Tibetans, the practice of meditation is simply the practice of preparing for death. You are becoming familiar with your mind so you will be able to see through the delusions that arise when your mindstream is no longer attached to a physical body.
Of course, you don't necessarily have to believe in rebirth to find value in these traditions. I have found the community aspects of Shambhala to be quite supportive, even when I had doubts, which in this tradition are expected to be plentiful, and are celebrated.
Happy to answer any questions!