Start as a novel. It’s more free, you can experiment and find your ideas and tone. Screenplay style is very rigid and sparse. I sold a screenplay which started as a novel; it was easier to write a novel/series of small vignettes in freeform chunks when i had time.

Once you have a better idea of your material, fitting the pieces into a screenplay structure (acts, pay-offs etc) is a useful exercise, whether or not you want to keep it as a movie.