I'm looking into doing something similar myself.
I think it comes down to your individual circumstances and running the numbers.
A 10-day pass is about $360 (I think, it's one of the ones you need to get through an Aussie
travel agent) and an annual pass can be $520 (with blockout dates, 10% off dining and merch) or $700 (no blockouts, 15-20% off dining and merch). If you think you will get $160 or more in discounts, then go for it! Accomodation alone might cover that, depending on how you go.
My calculation is going to be a bit more complex. I'm looking to visit
Disneyland AND WDW in the same trip, so I'll be comparing the cost of 10-day passes vs the premier passport.
Premier passport is $1029
Disneyland 10-day pass is $360, WDW 10-day pass is $470 = 830
So I'll need to find $200 in savings. I'm sure I'll save that much in merchandise alone
There's no point for my case (or yours) getting everyone annual passes as many of the benefits can be received by having one person in the party with an annual pass, and the park entry is met by the existing normal tickets.