[xiaq]

Installing other reader apps is exactly the point of having Android, and there isn't really a way to implement "just enough Android" to be able to run other reader apps. You have to have the whole package of an Android OS. I have a Boox device and use other reader apps all the time. There are also browsers optimized for e-ink (EinkBro is the one I'm using now) and it's great for reading books that are only available as webpages.

Also, developing your own OS is not necessarily cheaper than porting Android. Boox has been doing Android-based e-readers for a long time and their devices are usually on the cheaper end for the hardware. I feel it boils down more to UX rather than cost - do you want to let the user install whatever apps they want, or do you want to give them a more curated experience (and probably also limit them to your walled garden).

[xiaq]

Also by "reader apps" I mean "reader apps with their walled gardens", like Kindle, as opposed to readers for local files. Boox's builtin reader is OK, but I've used other e-readers whose builtin reader was worse and it'd be useful to have a 3rd party one for your local files. Ironically those e-readers are not Android-based.

I know you can de-DRM Kindle books, but there are other walled gardens that don't allow you to do that easily (for me it's some Chinese e-book apps).