Auster

Any with good hardware as long as you can switch to stable and community-made ROMs.

If you like reading books, comics, etc, maybe Librera Reader?

Dunno how it is on Google Play, but it's available FOSS and DRM free outside of it:
https://f-droid.org/packages/com.foobnix.pro.pdf.reader/

Same as the past 10 or so years, from what I can tell.

You also need to find instructions specific to your device on how to unlock the bootloader, and a TWRP specific to your device. From there, you do a clean flash through TWRP (erasing certain partitions and then installing the new ROM), and then, you should be good to go.

First, I'd suggest flashing a community system ROM. OEM systems are both very bloated, and stop being supported much earlier than community ones, so they're not ideal unless you need to use it with some banking app or the sort. And if you don't need Google's services (de-Google, anyone?), I strongly recommend going for a vanilla system instead of a Gapps one.

Now, as stated in MargotRobbie's comment, one good use is as a media player. If you can sideload stuff like VLC and Librera Reader, you should be covered.

You can also use it for some lighter gaming, if that's your thing, as there's plenty of emulators, wrappers and engine implementations for Android.

You can also use it for running servers, if you do this sort of stuff.

And if you like to test around with softwares, a spare Android device is pretty good to have.