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.
Fluent Reader★★★☆☆A simple, no-nonsense, modern-looking RSS/Atom newsfeed reader for Windows, Mac and Linux. Optionally sync with multiple services, but I’ve had issues with Nextcloud.
Fossify Calendar★★★★☆Basic calendar app that works with your phone’s local calendars. You can do all the usual things you want to use a calendar for on your phone. Doesn’t clutter up your schedule with ads or vacuum up your personal data.
Fossify Contacts★★★★☆Basic, privacy-respecting contacts app for Android that works with all contacts accounts on your phone.
NewsFlash★★★★★Clean, stable, fast, free, no-clutter and no-nonsense RSS/Atom newsfeed reader for Linux that optionally syncs with multiple services.
Nextcloud Bookmarks★★★★☆Online web app for managing bookmarks using your own Nextcloud server. I usually use it indirectly as the storage for syncing via Floccus.
Nextcloud Calendar★★★★★Self-hosted, web-based calendar that syncs easily with other apps and has completely replaced Google Calendar for me.
Nextcloud News★★★★★Simple web-based news reader for Nextcloud, easy to install and syncs with multiple desktop and 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 or local storage on your phone.
RSS Guard★★★★☆A solid cross-platform feed reader that runs on Windows, Mac and Linux. Extremely capable and customizable. Syncs with multiple services.
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.
Vivaldi (Web Browser)★★★★★Spiritual successor to the original Opera browser, this ultra-customizable web browser can open into a full suite for email, calendar, feeds and more – but only if you want it to.