Was the flight time that you eventually flew, similar or longer than what was quoted at the start of the flight?
Two things occur to me, when looking at the track. In the first instance, it sets you up to fly over Mexico, which is not an unusual way to go. But then it turns away, and tracks more or less along the border. That makes me wonder if there was some issue with the clearance.
Secondly, in the USA, because there is so much air traffic, when there is weather affecting routes, they tend not to let individual aircraft sort out their own act, but they just clear you on other routes that get you out of the way, but which may not be all that convenient to you. The very first time I took a 747 to JFK, the route we flew was related to the plan only in that it started and ended in the same place.
18:30 is a long flight. When I did the Dallas flight (only once) it was just on 17 hours. But, on that occasion, I had weight available for about another 15 tonnes of fuel, and a full flight. I don't think there's much cargo there, so straight away that gives a pretty decent amount of space for fuel.
Quoted maximum ranges are in specified conditions. Change anything, wind is obvious, but temperature isn't, and it has a great effect. Your flight would have been tight for fuel, but the system would have had to be able to generate a legal flight plan, otherwise a stop at Brisbane would have been on the flight plan. When flights are tight, the company will position a crew in Brisbane in anticipation of a diversion, with the final call as to whether it happens or not being up to the operating Captain. I know that they did so for your flight, as I received an SMS asking if I could help out on the 16th. I guess that scheduling haven't gotten the memo about my retirement yet.