I am planning a trip to Australia / New Zealand and I'm trying to squeeze the most value out of 4 global upgrade certificates by getting D1 upgrades on the long flights there and back.

I'm exploring lots of different itineraries and dates to find a situation that works, but I think ideally it would be a multi-city ticket that went from LAX-AKL (Auckland, NZ) and SYD-LAX, or vice versa, so that we only have to fly between the two countries once.

I see D1 upgrade availability both ways for LAX-AKL and AKL-LAX during our target month (generally a couple weeks in October, with maybe a week of cushion on either side if the tickets are there) but D1 inventory for the SYD flights is much spottier. I have found a few dates with availability, but it's hit or miss (both to and from Sydney) in spite of there being plenty of available D1 seats.

Is there some system for knowing when there will be D1 availability? I've noticed that you can get D1 upgrades for basically any date in June or July, but it starts getting spottier in August or September for the same flights. Is this because Delta is forecasting stronger demand for revenue D1 tickets in those months? Or will they eventually open them up for upgrade certificate availability?

What would be my best tactic for ensuring D1 seats on those flights? Buy one now, wait for the other? Wait for both? Lock in whatever availability they have now and plan the rest of the trip around that?

Thanks!