Out of curiosity, how far in advance does UA "usually" release PZ inventory?
("usually" meaning I'm already assuming it's completely variable )

I snagged a US-AU flight with PZ inventory on 2/21/24, but so far daily searches on a return either NZ-US or AU-US hasn't shown any PZ inventory whatsoever.
I flew AKL-SFO in August this year (booked in June in R) and upgrade cleared a few days prior to the flight.
Not so sure if taking the gamble in summer is as smart or if a random PZ spot will open up closer in.