Writing fantasy and sci-fi takes a very special idea in this day and age. It takes a unique question twisted up with a theme set in a world with beings who struggle, fail and through dumb luck and fierce will answer the question and find their path.
As a fantasy and sci-fi writer you get lost in your world, but you have to be careful not to delve too deeply that your story is never written. If you are spending so much time creating every detail of your world from the very beginning, you are probably never going to finish the book or series that inspired the world to begin with.
Why? because by the time you are finished building the world, which will take years, your story is finished in your mind and no longer peaks your interest. You have moved on to bigger ideas and the next adventure.
World building is a joy laced with satisfaction and challenge. Yes, you need to have a good idea about how your world looks, feels, sounds, evolves, and functions in most aspects of life.
I suggest getting down a framework, more than just an outline, but not as much as could be considered a novel in and of itself. Leave some wholes, you will need them later as your world grows.
Filling in all the blanks prevents you from having the flexibility to create something later to move your story forward. Leave things ambiguous. Leave questions unanswered. Leave ideas not fully explained.
Things happen as novels come to life and characters begin to interfere with what the writer had in mind. If you are not able to add things in because you have everything so locked in place from the beginning, you will have to go back and rewrite the entire novel. It’s not fun and everyone who has been writing long enough, has probably made this mistake.
Half written books live on all writers computers. It’s possible to dust them off and overhaul them after you leave them for a few years and forget many of the details. It is easier to just write the first draft, leaving question marks here and there and then go through a second and third maybe a fourth time and flesh things out more as needed.
You may intend for a novel to be a stand alone, and then a year after it’s published find yourself pulled back into the world for a sequel or two. If you haven’t left unanswered questions this can make it very difficult to finish what you started.