I developed https://qwyre.com as a PWA offline app-based epub reader. You're welcome to give it a go. It even imports links to epubs direct and stores them in your browser. There is no central online storage, no tracking, and here's the privacy statement https://qwyre.com/privacy-rights
Search doesn't seem to search inside the work I'm reading. Am I missing something?
I haven't implemented search inside the work. Only for searching the library (it's ultimately meant to be a self-publishing app, but that's a work in progress). It uses https://github.com/futurepress/epub.js/ Eventually, I'll get all its API working, but there's not much of a how-to guide, so it's slow.