1. I made a similar flight from Vancouver to Sydney last year, departing YVR at 2130 and arriving SYD about 0630. It was and all night flight which is IMO the best way to fly that route. BTW jet lag has a lesser effect flying east to west IMO. Whether it is dark or not on arrival will depend on time of year but most likely it will be light except for mid winter - June.
2. It depends. I'm a 6'1" 74 y.o. and was hestitant about my comfort but I was unable to to secure an upgrade. It was a Qantas flight and my previous Qantas flight was SNG/SYD earlier that year - it was my worst ever international economy class flight. However, as it turned out, the YVR/SYD was my most comfortable international, economy class, flight ever. I was able to stretch my legs out fully under the aisle seat in front where previously (SNG/SYD) my legs spent most of the time in the aisle. I'm not sure what the aircraft was but I think it was a 787-9 Dreamliner.
If you have trouble sleeping on long haul flights and can afford premium or business, go for it.
I have flown SYD/DFW business class and very glad I did. IIRC it was about 15.5 hours. I was lucky with that one by scoring a AUD650 upgrade at SYD check in.