DeferredForeignKey (null=False) whose instance value is typed with the related model.

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 = TypedDeferredForeignKeyField["OtherModel"]("OtherModel")   # -> OtherModel