DAVx5

Website
F-Droid Link
DAVx5 is an Open Source (GPLv3) CalDAV, CardDAV, WebDAV sync Privacy Tool installable on F-Droid. The service connects to a self-hosted Radicale server.

It's still a work in progress. The end goal is to have mobile devices perfectly synchronize calendars and contacts with all Thunderbird clients and other devices.