It never occurred to me, that people would use calibre to read books. I only use it to move books between devices (kindle →PC ⟷ smartphone) and to strip DRM. The stripping of DRM is actually my primary motivator to use calibre.

Readability is #1 priority

That’s arguable. Calibre is a database manager, not a reader. It has a reader, sure. But it’s an afterthought when compared to the rest of the program. The program is primarily aimed at people who have a reader and want to be able to manage their library. It’s days ahead of literally any other program when it comes to things like metadata management or managing multiple devices.

It’s sort of like saying that Notepad++ is bad at making Word documents. Like sure, it may be able to edit Word docs, but that isn’t what it’s primarily designed for.

Reader is very much a tertiary function of Calibre. It's an ebook manager and converter first, an editor second.