I think rather than Sabre's DAV, you're after their Baikal server.
https://sabre.io/baikal/ Sabre is also the guts behind Nextcloud's Contacts and Calendar dav sync, which you might want to look into, especially if you can see a need for sharing contacts among a group of people, as well as files, group chat and all the other features of Nextcloud. To bring it back onto topic, Nextcloud can also be used as a webmail interface to dovecot, of course. https://nextcloud.com/ P.
