Just how much of a pain is it to sideload books onto the Nook?

I really like the small size and the buttons and the bezels of a Nook Glowlight 4. Price is definitely appealing.

However, I've been doing this ebook thing for a long time and have bought tons of txts and epubs (and PDFs but I tend to use those on a computer) directly from authors, publishers, Humble Bundle, Project Gutenberg etc.

If I go down the path of a Nook, just how big of a pain is it to read what I already own? I don't mind fiddling, I just don't want to fight with it the whole time.