OCD About Your Library?

I have a confession: I have an addiction to cozy home libraries. My ideal library would have an old world explorer vibe, complete with a globe mounted in a brass-and-wood stand, a brass telescope, and a taxidermied mountain lion stalking a taxidermied big horn ram. It would also have a massive fireplace and at least three of the four walls would be lined with mahogany built-in bookcases. And those bookcases would be filled with books, yet have just enough shelf space left to accommodate any additions.

My current library attempts to mimic that dream library vibe, with mahogany lookalike bookcases on every wall, stuffed animals of the teddy bear kind propped on the bookshelves and the bookcases filled (to overflowing) with books. My Goodreads profile shows I only have 188 books, but I'm fairly certain that I have closer to 500 (many are too old to be scanned using their app and I will have to enter them by hand).

Ideally, I'd love to get all of them entered on Goodreads so that I could use it as a searchable database of my books, and then organize them on their physical shelves to match their Goodreads shelves, but until then, I have rudimentary organization going on. Horror over here. SciFi over there. Memoir up there. Children's books down there. Series are grouped together and placed in chronological (publication) order, but that's it.

And it works, in a sense.

Every once in a while, though, I stare at my bookshelves and know that as soon as I add a new horror book or if I purge some of the books given to me that I know I'll never read,  I'll have to reorganize. That thought is depressing when there are so many books to deal with.

So now my questions: If you have a glut of books, how do you organize them? If you organize by genre, how do you organize books that cross genres? How do you organize anthologies? And how much space do you leave per shelf so that you can add new books (or do you pack the shelves tightly and worry about that later)?