Welcome to FT! I think you are asking whether it's worth it to buy MQMs from Delta to get Gold before your trip to Brisbane so that you can access the lounge. If I am interpreting your question correctly, I don't think buying the MQMs from Delta is a very good value at all, especially if you have travel booked that will push you over the threshold anyway. I definitely wouldn't do it for lounge access only.
You would get a slight RDM bonus as a Gold vs. Silver. Is your LAX-BNE flight a Delta flight number or a VA flight number? If the former, how much did you spend on your ticket? Gold medallion earn 8 miles per dollar spent (vs. 7) which would offset a very small portion of the cost of the MQM purchase. If the latter, you would receive a 60% Gold Medallion bonus vs a 40% Silver Medallion bonus on redeemable miles (earned by distance on VA coded flights). In the case of LAX-BNE you would earn an additional ~1400 miles ($14 worth). Neither of those options are worth it, IMO.
Thank you for the reply and the warm welcome. Turns out I miscalculated based on my other flights that I'll be basically 2050 miles short of Gold after the last leg. I've been looking at options to upgrade my inbound leg (LAX-BNE) to Premium Economy or Business via VA's upgrade bid. Oh yeah and I did buy via VA so VA flight number. My offer from Delta is to get 12,500 MQMs for $1400 to maximize it or $599/$799 for 2,500/5,000 MQMs respectively.
It seems as though if I get upgraded to Premium Economy or Business it will add a boost to my MQMs. With that in mind, I'd might as well make the bid to upgrade. The lowest offer for Business is $1,050 at weak offer strength. Should I do that or go higher?