That would be nice if this can be ported to other eReaders like the Kindle and Kobo.
You can use a Kobo offline, see this guide [1]. A Kobo runs Linux under the hood, IIRC Android (probably some old version which is why I keep my WLAN offline and only allow on guest network. This also helps with battery). You can also install custom applications on it, for example Cool Reader [2].
A ReMarkable you can just SSH into. Which seems far more friendlier for customization, but its also a different price class and purpose.
[1] https://kobo-offline.virgulilla.com/
[2] https://www.mobileread.com/forums/showthread.php?t=222803
what? nobody mentioning Koreader? Light years ahead of whatever Nickel reader comes with the thing.
Sorry, I never used any custom firmware on our Kobos. You see, I keep them offline, so I'd care less about security vulnerabilities in them (plus if they go online, its via guest network). Also, I am very content with the software on our Kobo readers (Aura One and Aura Two). I do use Calibre + USB sometimes. What am I missing out on?