I don't think Kantian ethics is arbitrary.
At it's core, Kant is this:
The idea of duty - if you follow your duty, you are doing a morally acceptable action.
While this may seem simple, figuring out one's duty is a more difficult task.
Kant's framework was quite simple, for verifying if something could be your duty.
- It must not treat anyone as a means to an ends
- It must respect the free will of parties concerned
- If the action were universalised, would it cause any fallacies?
This is where Kant stops becoming arbitrary - he has just given three tests to see if an action could be morally good. And these tests do require knowledge based on the world.
If you need a more specific answer, don't hesitate to comment or something. I am happy to have a productive debate on this stuff