Over the past 15 years I've booked 8 Oneworld awards for myself & +1. I would have booked a lot more if it wasn't for Covid.
I've always tried to book the whole itinerary in one go as I hate paying the 5,000 points per person change fee. In practice, I've never been able to achieve my desired itinerary in one go. Either my preferred flights haven't been available at the first attempt or better options have come up closer to departure. So, I budget enough points to make two lots of changes and try to make as many changes/amendments as possible on each phone call.
Not sure if this helps
@ngocthuong but I would suggest trying to get the first few flights on the original booking (unless they are on one of those difficult routes where availability is rare). and then add a bunch of flights when the next difficult route becomes available. This is particularly important if yours is a long trip over a few months as the initial flights are unlikely to be still available by the time your last flights are released.
I should add that I am usually only looking for 2 seats. I did manage 4 seats in J on a Oneworld Award once, and once only! But that was before Covid. I haven't looked for a 4-seat itinerary since Covid but I suspect it is even more difficult than before; and it was extremely difficult then (as I was looking for J). Even in Y, the most difficult part of any Oneworld Award is finding seats out of and into Australia so if you do manage to find 4 seats on the same flight departing Australia, grab them! If you are OK with travelling Y you may get lucky but if you are hoping for J It is more likely that you will need to split your group into 2 lots of 2.
Just accept that it is going to cost you at least 328,000 points per person for a Oneworld Award in J (318,000+ 2*5,000 change fees). Obviously if you are happy with Y then things will be a lot easier and require less points (but you should still budget for a change or two.
It's worked for me over a number of years but as many members have noted, it ain't getting any easier.