Ok. I see your problem.
I googled a little bit more and it seems that you can actually make a blueprint node that accepts any (generic) struct and then operates on it using reflection:
So maybe you could make a generic ADD node that checks its two inputs if they are the same type and adds them if possible?
Not sure about the type of a return value of such node, that would probably not be possible to make in a generic way. But as a workaround you could accept your struct parameters as Ref and modify them directly instead of returning a new instance which would be what you’d expect from a normal ADD node.