Keep trying. Hopefully at some point it will stick. The breeding and grafting of these has gained traction. So someone really might develop a resistant version at some point. It’s a fun hobby to mess with them that I think is inherently rewarding for its own activity, rather than necessarily requiring any perfect outcomes. So, document or share your experiences. Someone else can learn from what you learn. And we can all work together to help keep this wonderful tree common throughout North America.