I'm just getting to grips with the DA14531 module and things are generally going pretty well ... I feel slightly limited by the number of GPIO's, but it should all work out ok for my current project.
One thing has been bugging me ... why didn't you route the flash SPI pins out to the module external pins? There are 6 GND pins so surely adding P0-1/3/4 would have been possible, and then you would have had some more flexibility with GPIO's and the ability to re-use the existing SPI bus rather than having to find three more pins for it!
Was it footprint backwards compatibility?