You don’t really need a custom UBlueprint class for that. In fact there’s very rare reasons you’d need it.
For custom Graphs you have to implement a custom Editor Toolkit class where you’ll base all your custom Editor tools on.
This is a very time-consuming task, lots of different undocumented classes you need to learn about to do it. I often come across the need for custom toolkits, but most of the time I’d rather invest my limited time somewhere else…