I thought if I waited to book the last few legs till I had the extra points the limit would then kick in?
That is correct. However if you were going to maximise the segments/routing of you OWA ticket (16 segments) you might spend 300k points on just 4 segments or so and have 10+ left to add.
Am I better to book all the legs at the same time even if some will need change of dates later as they become available? (Yes I know this will require all the points at the start)
Or better to book "half" the legs to start and then add the rest as flights become available for the dates I really need?
If you
have to change dates there's little point in booking anything, the seats you book are on a particular flight on a particular date only. There's no ability to transfer the seat to a different date. The only way is to cancel the booking for original seat and book the new seat (if it's available).
If you would
prefer different dates (but are happy to travel on the dates you initially chose) then you can book something suitable now and hope that options become available on the dates you prefer in the future. This would involve calling up and cancelling the original seat and booking the new seat (and potentially paying the 5k fee).
If you can see seats available on dates that you know you will want there's no harm in booking them. Normally you'd start with something out of and into Australia as these flights seem to see the most demand. With Qantas suspending bookings to Europe and USA there's not many options here currently so it may be best to hold off.
Also be careful as sometimes entire flights/months get booked out so your ideal itinerary may not actually be bookable even in the post-covid future. You indicated theres dates you
really need, I probably would only book once I could confirm they were available as otherwise you go to all the effort for nothing.
Good luck, Use the JAL tool to search for flights its 1 million times better than Qantas, and don't get tricked by the Emirates, Air France and KLM flights that show up on Qantas site they're not eligible for the OWA ticket.