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

The related model is referenced by string name (the FK is resolved later, once all tables exist), so its type cannot be inferred from the argument: specialize the generic explicitly. The related class is usually not importable here — that is the reason the FK is deferred — so import it under TYPE_CHECKING and use it as a forward reference:

if TYPE_CHECKING:
    from ...other_model import OtherModel

ref = NullableDeferredForeignKeyField["OtherModel"](
    "OtherModel", backref="+", lazy_load=False
)                                                        # -> OtherModel | None