I'm looking for a gradual transition from Reddit to Lemmy. To make things easier, I started to use Redreader. Man, I wish I'd done this switch earlier.
Redreader's dev has expressed interest on Lemmy compatibility, so when (if) this happens, I know which app I'll be using.
Privacy versus convenience... For what I know, the most common solution to your concern would be to setup a Calibre server and access it via your Android devices. But that's obviously a tad more complex than uploading EPUBs to Google Books.