ForeignKeyField (null=True) whose instance value is typed Model | None.

The related model type is inferred from the constructor, no annotation needed:

owner = NullableForeignKeyField(User)   # -> User | None

For a self-reference, the model type cannot be inferred from the "self" string: specialize the generic explicitly (as a forward reference, since the class is not defined yet inside its own body):

parent = NullableForeignKeyField["MyModel"]("self")   # -> MyModel | None