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