In a single-avatar game, what do the other main characters do?
From your description, you used the Avatars correctly.

At the outset of your solo game, the Avatars really only determining what crew member you are playing and what crew members will be available on the bridge. Those four crew members will define what strategies you'll use throughout the game.

As far as the other four main character Avatars that aren't you, their purpose is really only to take crew member specific strikes (defined on page 14 of the rulebook). Other than that, once you've set up the game, their impact on gameplay is very minimal.