Are there "safe" practices to follow before meeting one's teacher? I sincerely want to practice Tibetan Buddhism. It is a tradition which places great emphasis on one's relationship with their teacher. This makes a lot of sense to me. I plan to visit a Karma Kagyu centre near me when the coronavirus pandemic and associated quarantine measures subside and it is open to the public again. Are there any "safe" preliminary practices I can do in the meanwhile given that I have not formally taken refuge, received any empowerment or explanation and all my knowledge comes from literature? I am particularly unsure about whether it is a good idea to practice mantra meditation and mandala offerings without a teacher's guidance and my current view is that it is not.

For what it's worth I have been practicing breathing meditation and calm abiding (zazen) daily for a few months now as well as reading Buddhist texts and commentaries. But I don't feel like it has led to any non-intellectual understanding or insight yet.
Do not rush out trying to find a teacher. When you are ready the teacher will find you. Let the Buddha be your teacher and the Dharma be your medicine. When you do go to the center consider them as Sangha and learn how to engage them and see what you can learn. Be mindful of your own expectations and excitement for what that meeting will be like.

It is important to integrate the teachings in a heartfelt and sincere way. Intellectual understanding is important, but not nearly sufficient. Without actually practicing the teachings mere intellectual understanding can do nothing to help us. It is like being a chemist and understanding perfectly well the chemical mechanism of a medicinal drug. Unless one actually takes the medicine - not just understands how it works - there will be no hope of benefiting from it.

As for what practices to start out with... have you tried cultivating the four immeasurables as the Buddha instructed? The dharma is safe and you should feel free to practice it to your best understanding. I would begin with mindfulness, cherishing others, reducing or at least recognizing the non-virtuous minds, lay vows/practices and understanding what refuge is and means along with the four immeasurables.