That there are no surviving letters from Leopold now, hundreds of years later, when there was no reason to save any letters as there might be for a famous compo

That there are no surviving letters from Leopold now, hundreds of years later, when there was no reason to save any letters as there might be for a famous composer, does not indicate that there were no letters. Aside from that, it seems highly unlikely that there would be no letters just based on the fact that there is no known family drama or estrangement. I think you've made quite an assumption in your reasoning.