If you go for the Kobo Clara HD (Not the newer one), you can go around the storage size by replacing the internal microSD card (It is a bit of work, yes, but it works, and is doable)
Also, on Kobo, it is very easy to put KOReader on as well, so very much, comfy.
I have been running NixOS for a fair amount of time by now, and I can confirm, its a rad concept. Though I find NixOS the most comfy, becuase can run ZFS, and certain things, much more easily.
But yea, Silverblue, Kinote, and others, are nice as well.