tbh, I used to buy ebooks, and I had the same problem as with movies: the goddamn DRM is makign me crazy. So I gave up and went to library genesis and zlibrary. No problems ever since. I instantly find my books, put it in my sync folder and there they are on my eink devices.
Opening and syncing progress via koreader. Simple, efficient, open and no data lost.
I'll be back buying ebooks when they fixed their DRM BS.