I would take the Y option to BKK and waitlist for C. As others have said, you'll e ahead of upgraders and there is a very good chance it will clear.
I just flew SYD-SFO-LHR-SFO-SYD on an F award with only one segment confirmed in F, one in C and the remainder in Y. We cleared into F for every segment. Of course YMMV, but worht the risk IMHO.