To what extent is Kantian ethics only concerned with duty?

Kantian duty is self-legislation, not blind obedience

Kant certainly makes duty central: an action has distinct moral worth when performed because it is right, not merely because inclination happens to support it. The shopkeeper who is honest only to preserve reputation acts in conformity with duty but does not display a good will. This can sound morally austere, especially where affection motivates care. However, Kant's purpose is to identify a reason that remains valid when sympathy disappears, not to condemn sympathy itself. More importantly, duty is generated by autonomy. A rational agent tests whether the maxim of action could be willed as universal law and therefore gives the law to themselves as a member of a kingdom of ends. The humanity formulation reveals the substance of that law: persons must never be treated merely as means because rational agency has unconditional worth. Thus duty, freedom and dignity are mutually dependent. The genuine criticism is not that Kant cares only about rules, but that universal duties may respond rigidly when protecting one person appears to require deception or harmful consequences. Even then, the theory's concern is respect for persons, not compliance for its own sake. Kantian ethics is only concerned with duty to the limited extent that duty names the form of moral necessity; its deeper concern is the autonomy and equal status that make such necessity possible.