Ever since Edge dropped EPUB support, I've been trying to find a suitable "just open this EPUB file" replacement. Okular is janky and renders a lot of EPUBs incorrectly.
I've found Calibre's eBook Viewer the best I've installed as of yet, though I wish I could install just that part. Calibre feels a little heavyweight for what I want. But it does what it does pretty darn well, I think the lack of alternatives is because everyone has just decided to use Calibre.
If you're looking for an e-book reader in Linux, Foliate (https://github.com/johnfactotum/foliate/) is the best I've seen. I used to be a fan of Calibre E-book reader and had my own custom CSS for theming, but the redesigned e-book reader (I think it was Calibre 4.0) completely broke my setup. I don't know how it is these days, but it was extremely buggy and lagging when I tried some months ago. Even Bookworm (https://github.com/babluboy/bookworm) is a far better alternative than Calibre E-book reader.