Preinstalled Calendars and Contacts should be fine, on Calendars options menu you can use Online Account… and probably Contacts does the same once started with its first configuration.
I use an /e/OS Teracube 2e Emerald de-Google'd phone, it's got the Murena username@e.email setup with calendar / email / contacts / tasks / reminders / cloud storage setup.
I just discovered that my web host let me install nextcloud. In the nextcloud can be installed a calendar and an address book. I assume that there are applications under /e/OS that can access nextcloud and synchronize. Who knows?
Yes, Murena is nextcloud. I have just opened an account there. But I might not even need that if I have nextcloud with my hoster myself. At least I know him (a German, owner-managed company based in Germany).