I ended up going with Orijen. I bought a 7kg bag each of the 6 fish and Chicken adult. I decided to keep them separate though, at least for now until I make sure the girls dont have any issues with either. I will just be rotating days with the different flavours.
I also discussed the possible issue of food going mouldy with both my vet as well as the owner of my local pet supplies store, and both said there is no issue with having a bag of food last months as long as it is properly stored (i.e. seal the bag or use a plastic tupperware, dont store in your musty wet basement, etc.) - at least in our cool, dry climate there is little to no risk of this happening. Actually both even said up to a year is fine, although mine last about 3 months. But this is obviously dependent on the climate where you live, I will probably buy smaller bags in the summer months when it can be a bit humid.
Thanks for all your input!
Lady (1.5 year old tricolour) & Gracie (4 year old blenheim)
"Happiness is a warm puppy" - Charles M. Schulz