Features I need are being able to quickly open multiple documents side-by-side; a feature I actively don’t want is maintaining some sort of “library” for me.
Calibre's e-book viewer should cover your use-case just fine, and in KDE is quite easy to set that viewer as the default application for epub files.
I don’t want pagination for reference documents, for one thing – I want fast seamless scrolling.
Not sure if that’s still the case, but on macOS, it used to be hard to open more than one ePub file at a time using Calibre too – also an essential feature for research.
Finally, Calibre viewer edits each opened ePub by inserting a “last reading location” metadata file by default! It’s possible to deactivate in the settings, but I need to remember to do it for every new installation. It’s an absolute no-go as a default for a document viewer.
I don't quite understand: Highlight the desired files in the file manager and press enter? Tile the windows that open? I am missing something (obviously) ...