Delete the "Phone" part of a contact whilst keeping the "Google" part?
To answer my own question - here's a different approach to achieve this (Google-only contacts)


Ensure all required Android contacts are synced to Google
Export the contacts to CSV on a computer
Delete all contacts from the Android (that'll be both "Phone" and "Google")
Import the contacts from CSV to Google
Google will then sync to the phone


Kind of obvious in hindsight. Hope this benefits the community in some way!