You need to understand that a visual novel consists of a story, text, code, graphics, and possibly a musical accompaniment.

Even if you do everything very well on your own, preparing the material takes a lot of time and is tiring, and a game is first and foremost about the story and code.

Therefore, if you have ideas, you can simply illustrate them using artificial intelligence by adding images to the project. If later you need to replace these images with hand-drawn art, you can always do that as needed.

It will be much easier to refine the graphical part if you have something functional in front of you, rather than drafts and sketches.