I've done the flight 20+ times. Always in Y. The flight is definitely long. There's nothing you can do about that. One saving grace is that it's rarely full, so you may end up with a spare seat next to you. When I've been forced to fly in Y on that route I've used points to cover the cost of an exit row seat.
AA lounges will likely be what you are stuck with at DFW. Voucher system operates for drinks. Don't expect much, but the shower will be welcome.
I've spent quite a bit of time at KDFW - here's my advice.
Lounges - Personally, the AA lounge in concourse D is probably my favourite AA lounge - certainly one of the best (to clarify, all my flights have gone from D so I haven't tried the others - but certainly compared to AA lounges at other ports). For drinks, all basic drinks (including basic spirits, local beers) were free to all members - no voucher required. You can use your 2 vouchers for coughtails or premium drinks. Note - don't forget to tip $1 a drink even for the freebies. There's an awesome view from the lounge - but the food, like domestic Qantas clubs, is nothing to write home about. However, I always like to have a meal at TGIFs while I'm there so it's not a problem for me.
As you know the BA/QF lounges are off limits when flying AA, but seriously no great loss. I'm not a fan - I'd almost prefer the AA lounge. It's so
small and pokey. Drinks - at least spirits - were table service which I found really annoying (trying to take the edge off the 17 hour Y flight that awaits)
AA lounge in A is temporarily closed.
As for SYD-DFW - having flown it many times - it's hell in Y. Just prepare for and accept that. Do what you can to make it better - absolutely try to upgrade to Y+ or get an exit row. I've flown Y on the upper deck and it was very good - I had no problems selecting seats as SG so it might just be booked out.
DFW is so much better than LAX - I make a point of using it as my gateway whenever I'm flying to the US. If you get time explore the airport - there's some great stuff there.
