I don’t know if it’s what you are looking for, but if you are into self-hosting, look into Kavita. It is a FOSS digital book manager that has a fully featured web reader. It syncs your progress across all devices, and can handle pretty much all e-book formats.

If you aren’t into self hosting, and instead just want an app that can open epub, I’d look into Calibre, which has apps on all platforms and will manage your epubs and has a built-in reader. However, I would consider getting into self hosting if you are moderately tech literate. It’s a really fun hobby and I’ve got a setup for watching TV, movies, reading ebooks, digital comics/manga, and listening to audiobooks/podcasts. All from a used dell server in my basement, all automated, all running on FOSS software