Just confirming what some others said. Basically I have a Nextcloud setup where I sync all my devices though Nextcloud and can see Calendar, Contacts, and Tasks on Nextcloud, Android, and Linux (though this side could have been Windows for example).

The software I use for this is Open Sync on Android. This is a free fork of DavX5 which like others said also should also work. Then I just use my stock calendar, and contacts app, but I use OpenTasks for my tasks. On the other end I also sync with Thunderbird on Linux and can see my calendar, contacts, and tasks.

Little techie to all setup just because of all of the components. But otherwise works well.

OpenTasks: play.google.com/store/apps/details?id=org.dmfs.ta… Open Sync: play.google.com/store/apps/details?id=com.deepend…