If you use only "as" there it would sound as if you said "when" which is definitely not what you need in the context. "As" always ne is used in 3 cases: - in the meaning of "when": "She watched me as I was tidying up the room." - in the meaning of a reason: "As today is a national holiday, the shops are closed." - in the context like: "She works as a secretary." "Do as I say."

Hey! American here. I would not say "I'll get back to you as it's done." Not sure exactly why, unfortunately it's just one of those weird English things but I imagine it's because saying "as it's done" implies that it is completed already (as it is done = since it is finished) and then it doesn't work in that context anymore. hope this helps!