It's apparently rootable, although I haven't done that personally. It's Google Play certified so anything from the Play store works, and side loading Android apps works too. I use it with the open source KOReader app and in tandem with Calibre Web Automated. I did a writeup[0] with some details if you're interested.
[0] https://blog.eldrid.ge/2025/03/12/self-hosted-ebook-manageme...
I also wrote a short write-up about my experience with PocketBook devices and KOReader, for anyone who's interested: https://tc3.eu/posts/pocketbook-era-with-koreader/
The Remarkable 2 has an e-ink display but is rather underpowered as an e-reader. It does have an SDK for building apps: https://developer.remarkable.com/documentation/sdk
However, for reading technical docs or workshop docs in daylight, it's great.