I know this isn’t the type of answer that you want to hear, but I really love my kindle. It may not be open-source but it works and I can upload books to it from my personal collection. And the battery life is much longer than an ordinary tablet.
If you want to have something to tinker with then I have heard that the open book project is a pretty good build it yourself alternative.
This is true. And it’s also why I always recommend downloading steam through their website. They distribute their own Deb directly, and it auto updates.
Flatpak version is also okay but if you want to use a secondary disk then you need to know how to use portals (or the Flatpak configuration tool that I can’t remember the name of).