First Kant was an enlightener (“Aufklärer”), then he was an idiealist because he stopped or overcame the era of enlightenment and started the era of the post-enlightenment idealism (“post” because there were some idealists long before Kant, for example Leibniz). In any case, Kant was the “father of the modernity”. I guess that, if you had lived at Kant’s time, you would have tried to prolong the era of enlightenment by saying “yes” to the question “is causality true?”. Kant referred to the epistemology, to the knowledge, thus also to causality but not so much to the metaphysical question of the truth of causality. After Kant the question of a true causality has been occurring again - similar to the time before Kant but (and that is the huge difference) by referring to Kant, thus not without Kant’s philosophy.
A rational ontology includes causality, yes. But does it really make the causality true? One could also say that we accept the world as the truth but do not know whether it is the truth or not.
According to the question of truth there are four answerse possible:
- There is truth.
- There is only truth outside of the (brains of the) subjects. This answer is philosophically called objectivism.
- There is only truth in the (brains of the) subjects. This answer is philosophically called subjectivism, as an extreme form: solipsism.
- There is no truth.
So we have one absolute affirmation (see: 1), two relative affirmations / negations (see: 2 and 3), and one absolute negation (see: 4).
It seems that no one of them can be proved or disproved.