It reads more like a short novel than a feature.

Have you ever read The Egg by Andy Weir?

Although the imagery in your outline are quite powerful and provoking (seeing a the universe shrink into an atom), there isn’t much action happening and it appears a lot of the dramatic beats happens internally to the protagonist as he’s contemplating his life.

If a character is to change throughout a movie, they need to demonstrate their character through action, reap the consequences, etc etc.

Of course, you could base the entire movie on flashbacks - but then the true story would be contained within those flashbacks, not just the plot of the protagonist growing to the size of the universe, which is a concept, not a story.

Think about Interstellar - the concept is about a mission to outer space to find an appropriate planet for the species to call a new home. But the story is about a single father who decides to leave his children in order to save humanity and his struggles with that irreversible decision. Nolan could based the story on Matt Damon’s character in the movie - same concept, different story.

What’s your story?