Kobo devices work out of the box, and they are very hacker friendly, a good middle ground. They run mainline Linux with a custom UI, but you can change that to other systems, e.g. PostmarketOS (an alpine based linux distro) or install more fully featured reader programs like Koreader. With some hacking you can use the devices without a Kobo account. Some devices run from a simple sd card, so you can easily grow storage size, and easily backup the whole system. Most devices have hidden a serial port inside, with some soldering you can connect peripherials!

I have a clara hd, I use it as a tactical glide computer with xcsoar,, with an external serial gps and barometer