I've used https://standardebooks.org/ to pull nicely formatted Project Gutenberg books on any e-reader that supports a browser (in my case, Boox).

Technically, I can also just directly pull the epub from Project Gutenberg, but sometimes the formatting leaves a lot to be desired.

Once you get an e-reader that runs a semi-capable OS (ex - stock android, even an older version), it's hard to go back to something like a kindle.