This👆

That's why I referred to it as "plain": it's just the reader. And it is mostly made to replace the original reader software on eBook devices, but you can also use it on desktop if you want.

The question is if you are looking for just a reader, or a library manager. But you can also mix the two, so use calibre fo manage the library and have it use koreader to open books

Librera is for people who are Android users

I'd reword this a little, to

Librera is better for people who are Android readers exclusively

Because if you're the type who uses a dedicated e-ink device for reading, also having the same application and book library on your phone has some obvious benefits. But if your phone is your one reading device, you'd probably try the Android version of KOReader and be like "wtf is with this UI?"

I use KOreader on an Android phone because it's so configurable and allows me to read efficiently on an AMOLED with white on black text and a very minimal, space efficient interface. Also the large amount of style overrides allows one to fix even the most badly formatted epubs without editing or processing them in Calibre.