for reusing code between states, did you try impl blocks that are generic over the state type parameter?

for

    struct State<T> { state: T }

with possible states

    struct A { id: Uuid }
    struct B { id: Uuid }

use a trait

    trait HasId {
        fn id_mut(&mut self) -> &mut Uuid;
    }

now you can impl over both A and B

    impl<T: HasId> for State<T> {
        fn new_id(&mut self) { *self.state.id_mut() = Uuid::new_v4() }
    }

simplistic example but enough to communicate the idea hopefully.

generally, macros makes this kind of thing ergonomic so you can generate the trait implementations and so on. otherwise it's a lot of typing.

don't understand what you mean about making callbacks to other APIs during transitions.