My standard eBook viewer is a 7" Android tablet. I use that rather than an eInk device because eInk is monochrome, and I need color support. Too much of what I read uses color in things like embedded images, and gray scale conversions are painful. (There actually is a color eInk display, but it's 12 bit, which is inadequate for my purposes, and only one obscure Chinese vendor had produced a device using it the last I knew.)
The books are actually stored on a 32GB microSD card, so support for external storage was another requirement. In practice, I pop the card from the tablet, plug it into an adapter, and drop books from the master library on the desktop machine directly onto the eBook directory on the card from the host. There are other ways to do it, but it's quickest for me.
I use the open source Calibre program to create and maintain the master eBook library, and use Calibre to transfer books to the card, and it creates the needed sub-directories as required. (Calibre stores books in directories by author name, and more than one book by an author will go in that directory.) Calibre can also convert between supported formats, and I do so with a caveat - conversions from PDF usually fail, and I've stopped trying. Too many things about a PDF just don't convert, and the results are painful. See
https://calibre-ebook.com for info and downloads.
To view the books, I use the open source FBReader for Android applicat6ion. FBReader gets the nod for support of multiple formats. It dispalys ePub, AZW3, Mobi and FB2 files among others native. It displays PDFs, CBR/CBZ (Comics) file, and DjVu files via plugins. I prefer ePub, and convert Mobi and AZW3 files to ePub as the storage format, but for the most part, I don't have to care what format a book is in. FBRFeader does
not handle books with DRM, but I don't get such things and don't care. FBReader is highly customizable, and I get a page display about like that of a mass market paperback. FBReader has a free version and a payware premuim version. I happily bought the latter. The Premium version builds in the Bookshelf view that requires a plugin in the free version, and builds in PDF support as well. See
https://fbreader.org/ for info and downloads. It's also available on Google Play.
My device is LCD, but good luck on finding color support in a reader that doesn't use it. The advantage a lot of people like in eInk readers is battery savings. Once an eInk display is painted, it requires no power to maintain it, where an LCD requires a constant trickle to keep the screen refreshed. Folks with eInk readers report going for weeks between charges. That's nice, but I need color and simply made it a reflex to charge my various devices regularly. My eBook viewer tablet tends to live on the charger overnight when not actively used.