First, I assure you that your translation is already good. You understand the sentence structure perfectly. One might question if your word choices of "disturb" and perhaps "this" are best, but it is good that you understand that is a relative clause. If you had, as some would, placed a "mental comma" after the , it would have cost you a lot.
Regarding vs. , it is often a subtle choice. While I should not make a hasty comment by reading just a few lines from an entire book/story, we tend to use // to keep a certain distance between the narrator and the events decribed by him/her. "Objectivity" would be another word for "distance". This is usually the same even when the narrator is the first-person.
> "~~~ by me, who is speaking (as) matter-of-factly as that."