I'm a bit confused as I thought that BA009 went through to Bangkok - you then need to change to a Qantas flight to get to SYD.

There are lots of threads and trip reports of what to expect.
Lounges: T3 is very pleasant. If you are going via BKK I found it very underwhelming - but good showers and far less crowded than the SIN lounge which gets very crowded as all the BA and QF flights transit through around the same time.

Only you can answer if F is worthwhile for you. Again lots of descriptions available on FT to help you decide You will be spending a long time on the plane and if you get a good crew the dinner etc can be very good and can fill up a good bit of time. The entertainment system is very good on NF but dissapointing if you get OF. Likelihood of getting a noisy baby / young child close to you might be higher in CW (my adult daughter had a very disturbed night in CW going through to SIN recently - no guarantee it couldn't happen in F).

If you have not been to SYD before have fun (and I think you will fund it beats a plane trip in any class)