I don't know if they are the best option, but Kobo stands out in terms of being a practical reader and being hackable.
A nice feature on most models prior to the Clara HD is an internal uSD card. (This is different from the externally accessible uSD slot found on some models.) Pop open the case, remove the card, make an image, and you can be assured of restoring it to a working state without any fancy tools. With the uSD card in a computer, you can examine it and modify it. The models that I have cracked open also have a place to add a header for a serial connection, though I haven't tried using it.
Kobo does not appear to have any interest in locking down their devices. Software updates are shipped as zip compressed files wrapping a gzipped tarball. That tarball is simply a directory tree containing files to add or replace. Many third-party modifications are distributed in this manner and you can do your own modifications without ever cracking the case open. (Though I do suggest opening it up and making a backup before modifying it.) As far as I know, there is no protection on the proprietary software components and Kobo hasn't said anything about third-party patches. This has been the case for nearly a decade now, which is why I lean more heavily on the word modifications rather than hacks. Most of what can be done relies upon an developer's knowledge of Linux.
There are also various projects that you can learn from. Some provide tools to make developing software for the Kobo easier, while you can examine others to figure out how to setup a toolchain. I have seen C, Rust, Lua, and Python based projects for Kobo readers.
The Kobo development forum on the Mobileread forums is the place to check out for more in depth information.
I presume that the internal sd card would help the longevity of the device as well. Like someone mentioned, they upgraded their storage. You could do the same thing, if after say 5-7 years, the card starts degrading and the reader starts slowing down.
A nice feature on most models prior to the Clara HD is an internal uSD card. (This is different from the externally accessible uSD slot found on some models.) Pop open the case, remove the card, make an image, and you can be assured of restoring it to a working state without any fancy tools. With the uSD card in a computer, you can examine it and modify it. The models that I have cracked open also have a place to add a header for a serial connection, though I haven't tried using it.
Kobo does not appear to have any interest in locking down their devices. Software updates are shipped as zip compressed files wrapping a gzipped tarball. That tarball is simply a directory tree containing files to add or replace. Many third-party modifications are distributed in this manner and you can do your own modifications without ever cracking the case open. (Though I do suggest opening it up and making a backup before modifying it.) As far as I know, there is no protection on the proprietary software components and Kobo hasn't said anything about third-party patches. This has been the case for nearly a decade now, which is why I lean more heavily on the word modifications rather than hacks. Most of what can be done relies upon an developer's knowledge of Linux.
There are also various projects that you can learn from. Some provide tools to make developing software for the Kobo easier, while you can examine others to figure out how to setup a toolchain. I have seen C, Rust, Lua, and Python based projects for Kobo readers.
The Kobo development forum on the Mobileread forums is the place to check out for more in depth information.