For a sci-fi story or any story not set in the real world, world building is key to understanding the story.
Branching does make me feel burnt out when I have written over 6,000 words, but the average play through is 400 words and the game is very short despite the work I put into it. Look at games like Yugen and Guenivere that have little branching, but are great.
The characters should usually get a majority of the focus as people love interacting with characters and romances.
Continuity is important if you might want a sequel on the future or if the game is not a comedy, but is usually not that important.
If it’s a slice of life or comedy game, the plot isn’t too important.
When I write, I first form a basic plot and then populate the world with interesting (at least I hope) characters and see how they work with or against the plot. I place the MC in the world and try to inform them enough about the world so that they can make semilogical choices, but not enough to overwhelm them or remove all twists. I create the mechanics that allow the MC, NPC, and world to interact, which are usually relationship bars as my story is character focused and then I tweak things as needed.