I also use Google Contacts as my Truth and sync them to macOS and iOS Contact via the default Google account.
On macos I use the web interface to Gmail and Calendar so the only reason I need to sync Google Contacts to macOS Contacts is for my greeting AppleScript. Unfortunately, in my experience, this doesn’t sync reliably and some failures are completely reproducible. For example, after a contact has been created on Google and synced to macOS without a birthday, adding a birthday in Google will never sync to macOS. I think that syncing has become less reliable as the number of my contacts passed 6K.
On iOS it seems that all my email apps, Spark, Gmail, and Mail—each can do essential things that the others can’t so I use all of them—use iOS Contacts so syncing is very important.
It never occurred to me to sync them via CardDAV. Thank you for the suggestion.