No doors on QF J (nor F). I flew FJ NAN-LAX a few years ago and IIRC the J seats were slightly sloping flat. QF J seats are flat (not sloping). AFAIK all QF 380s have the new J seats but if not then pray you don’t get one with the old J seats where the foot rest end slopes down due to heavy pax and crappy manufacturing /design. The remedy is to shove a cabin bag under the foot rest which restores the correct profile. QF can and have done late substitutions of a 789 for the 380 - be prepared. FJ has a very small fleet so if one aircraft goes tech then this can impact schedules (but QF is also notorious for tech aircraft killing on time running). If stopping in Fiji and wanting to escape the airport be aware of (any) current COVID testing requirements. If thats no problem then its easy to rent a car and go exploring - just don’t expect fast driving if you are running late for your return. If you are OWE by the time of your travel, you may have problems trying to enter the SYD Flounge if on a FJ code.