Backstory writing is a great tool for you the writer. However, the backstory is not the story you are telling. The current story is.
You have done the right thing by starting with the action. That is a great way to start a story. Not the only way but a dependable way to get the ball rolling.
When it comes to showing your homework, that is giving the backstory for your characters, if you “drop it in” it will almost always be wrong. The reason is that you have to stop the current story to tell another story. Generally, this is known as doing an “info dump” and it is not a good idea.
Instead, drip-feed, the backstory into the main narrative only at the point hat it first becomes relevant. That way you will not slow the current story to tell readers what they need to know and what you do tell them is instantly relevant to the story they are reading and, therefore, interesting.