Florisboard is a nice minimalist alternative to GBoard. Offers decent theming and lots of settings for haptics/layout, makes a very quick/easy replacement on new devices.

NewPipe is a minimalist frontend to YouTube that lets you download videos and use PiP/background playing without paying for Premium (or logging into YouTube, for that matter)

Bromite offers a de-Googled Chromium experience for mobile users, pretty barebones and basic.

Glider is an awesome FOSS Hacker News client that I use for checking headlines on the go. Hugely recommended if you're an Android user!

The last big one I use is KeePassDX, which is a password manager integrated with the open KeePass standard. All my passwords get locked up in a nice encrypted, portable file I sync across my devices.

Now, I also use the Play Store for stuff like Discord and Spotify, and iOS users probably would still use the App Store if Cydia became officially supported overnight. That being said, having FOSS replacements for shitty defaults saves the modern Google-based Android experience for me. If they removed sideloading, I'd probably jump ship to a more minimalist fork.