Nickel, the stock firmware on Kobo, is pretty decent. KOReader is better, though. The Kobo is a pretty hackable device. I pair my Kobo Clara BW with BookFusion, which gives me portability between any KOReader device, Mac, Windows, iOS, and Android.

You can already easily install koreader on the Kobo. You can even combine it with Tailscale to get your local opds server