Tradd, I know that you can do this and do it well!! I was not criticizing you but being an objective voice. I am a booster for your efforts.
I understand from your posts that customs is small industry in numbers but with a big role!
You have mentioned that his is a small business that has grown very well. He did his research and hired you (!!!!) with your extensive knowledge and experience knowing that he was trusting his reputation, his business and his livelihood for his family on this decision.
A big no-no as you called it triggered a strong understandable response from him. It is now water under the bridge, can't be undone. You did well to acknowledge it. It is a learning opportunity for both. Build on that! You are part of a team.
What can you do to increase your knowledge and understanding of the business? If you were running your own business in this field, what could you learn to do it better? I frankly know little of the customs industry so can offer no suggestions. Find additional ways of supporting his business by walking in his shoes. He needs your services and was transparent about this. With Covid and working from home, can you find a challenge this winter in the industry that you can explore further? Spend the money to do so if necessary.
You said at the outset that you were looking for new experiences and opportunities for growth in your field. You go girl!!!!