Remove the DRM from your ebooks; convert to an open format like epub; read it with open source software on anything you like without tracking.
If you don't do this, acknowledge that you are renting a reading experience -- and pay what that's worth to you, as opposed to the price of owning a copy of a book.
You've missed the point. Something like a Scribd or Oyster subscription use an app and its own eBook format. The books are cached or streamed, not downloaded permanently. You are paying to read, not to own.
Calibre is the best ebook manager. It's the iTunes for ebooks.
On Android, the FBReader.app is quite good, however you probably will want to tweak the options to get it to look as nice as the Kindle.app looks "out of the box."
On iOS, the Bluefire.app is the best of the unaffiliated apps, but I preferred to open .mobi files in the Kindle.app.
If you don't do this, acknowledge that you are renting a reading experience -- and pay what that's worth to you, as opposed to the price of owning a copy of a book.