I'm basically married to my e-reader app, but the distinction between online and offline needs more focus. If you crave a vast library you can sample instantly, something like Kindle Unlimited is tough to beat for sheer volume, even if the curation is hit-or-miss. You download titles to the app and they're yours until your sub lapses.
For a more focused, almost old-school feeling, I lean into Libby with a library card. The selection depends on your local system, but it's completely legitimate and everything downloads. The waiting lists for popular stuff teach you patience, weirdly enough. Sometimes I find a random translated novel there I'd never have paid for, read it offline over a weekend, and feel like I got away with something.
My phone's storage is a graveyard of apps I tried once. The one that stuck, aside from those, is actually Moon+ Reader Pro for sideloaded EPUBs. It doesn't have a store, so you need to source files yourself from places like Project Gutenberg or Humble Bundle, but the customization for reading comfort offline is unmatched. It feels like my own private library, organized exactly how my brain works.