Kobo are giving you root access with telnet from the start. You can flash modded firmwares, change the backend servers to phone your own calibre-web instance, install ssh, koreader and even a tailscale vpn on it. They even have UART pins labelled on their board. These are amazing devices to play with. And they read pretty much everything you throw on their storage: epub, cbz, cbr, pdf..
I bought a pretty early Kobo like 10+ years ago and found myself very underwhelmed with the software situation. I sold it and went back to my kindle. Maybe it's time to take a second look. The hacker in me's interest is piqued. Frankly though, the biggest thing that keep me on Kindle these days I suspect are hard to replicate. For example, automatic audio book/kindle sync where I can switch back and forth, listening…
Trashy sci fi is my vice.