I gave Calibre a brief shot and was immediately put off by how big and clunky it was. I'm sure it would have been perfect if I gave it more of a shot and spent time tweaking.
Kavita has been my solution for the last probably 7 months and I'm loving it. I don't need anything outside of "put book in place" and then "Open Kavita, see book" and it has been perfect for that. It's essentially plex but with books in terms of how using and maintaining it has been.
I solved that for myself by making genre folders and just tossing books in them haphazardly as I download them.