Geary★★★★☆Really lightweight but still modern, so it’s a good choice on lower-end hardware. Basic IMAP features, good for most day-to-day email use.
K-9 Email★★★★☆Classic email app for Android: No frills, no ads, no tracking. Supports multiple accounts, phone-to-tablet layouts, and dark mode.
Nextcloud Calendar★★★★★Self-hosted, web-based calendar that syncs easily with other apps and has completely replaced Google Calendar for me.
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.