DAVx5★★★★★Background app that syncs your non-Google cloud accounts with your Android system calendars and contacts.
Floccus Bookmarks Sync★★★★★Very flexible, syncs across many different desktop browsers and mobile devices, and for privacy it can run on your own server or encrypted on another cloud service.
NewsFlash★★★★★Clean, stable, fast, free, no-clutter and no-nonsense RSS/Atom newsfeed reader for Linux that optionally syncs with multiple services.
Nextcloud News★★★★★Simple web-based news reader for Nextcloud, easy to install and syncs with mobile clients.
Nextcloud Notes★★★★★Simpler than Google Keep, more private, with human-readable data that syncs quickly and cleanly with your devices.
OpenTasks★★★★★Simple to-do list that works great with a Nextcloud server.
Thunderbird (Email and Calendar)★★★★★Stable, capable desktop email application, works well with multiple accounts including Gmail, Nextcloud, easy to set up and use but with advanced settings when you need them. FLOSS.