DAVx5

★★★★★

Works great. You only have to set it up once (unless you’re adding or removing new lists/calendars), and then it seamlessly syncs your address books, calendars and tasks between your CardDAV or CalDAV server and your Android device, and you can use them with any app on your phone that uses the system calendars and contacts.

I can use my Nextcloud contacts with K-9, GMail, the Contacts app, and even for phone calls. And I can use the calendar with anything from Simple Calendar up through the Google Calendar app, and it all syncs up and down.