How do I build a repair business on trust?

You have understood the real foundation: your diagnosis is honest whether or not anyone could ever catch you, and that trust is the entire business. Many shops chase the quick sale and wonder why customers never return. You have seen that the return is where the money actually lives. Trust is slow to build and fast to break, which is why it is worth so much. Every honest call you make, especially the ones where you tell someone a repair can wait, is a deposit. Over years those deposits become a reputation that no advertising can buy and no competitor can easily copy. Keep doing the thing that feels like it costs you: tell people the truth even when a lie would pay better today. Show your work, explain your reasoning, let them decide with clear eyes. Do that consistently and the town starts sending you its trust. That trust is not a nice extra on top of the business. It is the business.