It seems to me that my expectation of CardDAV was wrong, basically the nextcloud address book(s) act as additional address books on the phone just like they are additional Calendars, so what is on the phone stays on the phone while what is on Nextcloud or created on the phone set to Nextcloud will sync there.
Iām now digging through contacts.db to see if I can modify it to quickly have all my contacts migrated to Nextcloud.
I think my mistaken perception came from a time that I setup a google account and forgot to make it sync one way and e-mail only and thus violated the privacy of the part of my address book that got uploaded, which does suggest that this could be possible for Nextcloud too.
Lastly from my initial inspection of contacts.db it looks like by not specifying a single address book for the phone to use items will get duplicated since Nextcloud shares some automated address books like āRecently Contactedā, thus it may be wise to adjust the Advanced settings and point the phone only at a specific Address book or (if a Jolla dev is here) maybe add an interface similar to the Calendars where you chose which of the address books should sync.