It’s as open as most Android brands. I don’t use any of Boox’s services or apps. I installed F-Droid and use open-source apps from there. I use Librera as my ebook reader, with Syncthing to sync my book library between my desktop, ereader, and phone. It’s possible to set up the Play Store but I don’t bother, personally.

It’s not a 100% smooth experience but I’m very happy with the F-Droid compatibility. I absolutely refuse to get locked into a walled garden.