I don't think the deal was about "until Ciel felt he had finished everything". I got the impression that the terms of the pact were more specific, even if they didn't explicitly say so in the anime. Maybe it was something like "until everybody who was responsible for the death of my family has been punished"?

I wasn't very satisfied with the ending of the anime, but I have to admit I do like that part about the pain. As far as I understood it, Sebastian offers to be as gentle as possible and Ciel refuses, because he wants the "pain of his existence to be carved into his soul".
To me that meant that he wanted to give himself to Sebastian completely, with everything that made his life/his existence. No pretence here - pain was part of his life and so it has to be part of his death, too. Without that pain Ciel's soul wouldn't be complete.