The QR 18:45 arrival is ideal. Less immigration & customs queues. By the time you are at the hotel it’s 20:00 - with guaranteed check-in.

You can have a meal, a workout, a night stroll and go to bed at a reasonable hour and you should be on the timezone faster than the morning arrivals.

Re:A380 - the seats are still good and you have the bar area to chill and break the trip up. Normally there’s a very good cognac there too.
Dine on demand for food plus Al Mourjan/Gardens access in DOH prior. QR hands down for me - especially if direct to SYD is preferred.