Updated from the App Store for the Z370 Prime P, then system rebooted into installer. Installation was very smooth both for the update and for the Nvidia drivers. Sound drivers working okay too. Only concern was that when we tried to process very large files with Photoshop CC, the system crashed. Bear in mind this was before I updated to the latest Nvidiagraphicsfixup and Lilu. It seems stable now, will post if anything occurs.
Also a note for those who've experienced crashing on the Z370 Prime boards - you may need to move the RAM to Slots A1 and B1 instead of A2 and B2 in the manual, and then set them in Clover as Slot 0 and Slot 2.