Thorium Reader is an absolute pleasure to use. Open-source and regularly updated.
https://www.edrlab.org/software/thorium-reader/ https://github.com/edrlab/thorium-reader
https://www.edrlab.org/software/thorium-reader/ https://github.com/edrlab/thorium-reader
Not saying there shouldn't be an app for those who want it.
If you want to build a good ereader, you either use a web browser or you reimplement one. And you have to support things like https://developer.mozilla.org/en-US/docs/Web/CSS/hyphens, rtl, mathml, and—well—the rest of the nontrivial stack.
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.
Your options are A) web view, B) crappy RSS reader.