[theandrewbailey]

I second LineageOS. I put it on my Kindle Fire 7, and it became fast enough to be usable for the first time. Flashed it again without Google Play services, and it got a bit faster. I use it to read email (Fastmail), listen to music (VLC), and occasionally watch youtube (NewPipe). I can recommend FDroid and Aptoide to replace Google Play.

[smoldesu]

I ran LineageOS on my Nintendo Switch for a few months, and it was uncannily stable. I had accelerated video playback, ran Obsidian and got all my other apps working mostly fine. A good LineageOS build can really change an older device for the better (but my Switch runs OEM firmware today because I despise fun).