Davie - Did you manage to fix this? - I had a go at Viveza 2 and it works. (Workflow is through Photos -Edit, Extensions Affinity Photo - plugins - NIK)
Its all good except for HDR .. Also I would love to have Luminar (flex/2018) working as a final touch.. No go yet:(
Steps:
Removed everything Nik related I could find via Terminal (feks
find . / -name "*.plugin", find . / -name "*.efex", find . / -name "*NIK"
Reinstalled to a new location
"initialized" the Nik apps by starting them standalone and opening an image
Configured Affinty Photo as shown attached.. Interesting to note that Vivesa and Silver Efex status is unknown - But they do work..
Versions
Nik 5.0.2
Mac
10.14.5
Affinity 1.7.1