Firstly - thank you to Mwenenzi GinFizz and pandaperth and others for your helpful tips above on my proposed RTW itinerary. For a few different reasons we have largely decided to stick with our current plan. The AA RTW desk has validated this routing and is currently holding the following booking for us:

OSL-DOHx-SGN-PNH - Qatar Airways
PNH-HAN - surface leg
HAN-HKG-AKL - Cathay Pacific operated but booked on a QR code share
AKL-SYD-HNL - Qantas
HNL-DFWx-YVR - American Airlines
YVR-LHR - British Airways
LHR-DOHx-DXB - Qatar Airways
DXB-LHRx-OSL - British Airways

The pricing has also just come through - the base fare (£4,178) and lots of various taxes (total of £714) are broadly what I expected but the "other taxes and carrier imposed charges" are £1,998! Whilst I expected a certain level of taxes and carrier charges I am still quite surprised at how high these have ended up being. There are 3 adults and 1 child on this booking so the total carrier charges end up being a huge total amount for us.

Curiously the "other taxes and imposed charges" for my 10 year old son are showing as an even higher £2,760.80 - which makes me think there must be an error at least in the child fare and perhaps also in the adult fares that have been presented to me?

I am happy to change airlines etc if that helps the cost - also happy to adjust the overall itinerary also if it saves a meaningful amount of money (but do still want to ensure we get to Vietnam, AKL, HNL, YVR as a minimum - ideally also preserving DXB at the end which would be a separate trip later in the 12 month period for us). I have one spare segment left in the current booking and am also happy to drop SGN-PNH if it is helpful to have a further segment available to change the routing to reduce carrier charges.

Not sure if this is relevant at all - but the AA desk insisted I pay in GBP as I am based in the UK - I had expected to have to pay in NOK (as it is an ex-OSL routing) and am happy to do that - wasn't sure if having to pay in GBP added to the overall cost in anyway? I had assumed the FX translation from NOK to GBP would be at normal FX rates.

I'd hugely value any advice/tips on how I might be able to improve the overall cost.