Standard Ebooks (standardebooks.org) has beautifully formatted public domain EPUBs. There are also 70K+ free books via Project Gutenberg and similar sources.
I built an app that bundles access to 1.5M+ free public domain books from multiple sources, with OPDS support for Calibre/Kavita if you self-host. Free on the Mac/iOS App Store: getbookshelves.app
Cool project. The reading position sync problem is real.
I've been building an EPUB reader for Mac and iOS that syncs via iCloud. Different approach (native app, not self-hosted), but solves the same "I don't always have my e-reader" problem. Also has OPDS support for connecting to Calibre/Kavita servers.
Not open source but free to try: getbookshelves.app
iCloud-based sync might be simpler than self-hosting for this. I built an EPUB reader for Mac/iOS that syncs reading position across all devices via iCloud automatically. Open a book on your Mac, pick up on your iPhone exactly where you left off.
Also connects to Calibre/Kavita via OPDS if you want to keep your self-hosted library. Free on the App Store: getbookshelves.app