I have a bad news for you, recently I was interested in how could I write a good QML/Kirigami based ebook reader and looked at all the existing Linux epub reader and each one was using a webview for the epub rendering. But they usually try to integrate better with the rest of the app and your DE so you just don't see it.
EDIT: the only epub reader I know that doesn't use a webview is Okular and the rendering using QTextDocument basic html support is horrible.
EDIT: the only epub reader I know that doesn't use a webview is Okular and the rendering using QTextDocument basic html support is horrible.