ONLYOFFICE has released two new plugins for online editors - Mendeley and Zotero. These services allow searching references in the libraries and inserting them in the bibliography with the desired citation style.
This still depends on the Zotero server API right?
Zotero actually uses WebDAV, and I always thought it would be neat to have your own Zotero server in Nextcloud, but someone would need to write a NC app for it.
Indeed, it works with your account in the service.