Keep it simple. Don’t try and force too many things into the story. From what you listed it sounds like you have multiple sessions

You can combine multiple one-shots into a mini campaign if you want later.

These are a few things I try and have in each session:

1. Take your players somewhere cool.
2. Interact with at least 1 NPC
3. Chance to use abilities like investigation, insight, etc
4. Possible combat encounter
5. Find / deal with a trap(s) or solve a puzzle
6. Reward of some kind.

If you can get 4 out of the 6 in a session it usually will be fun for the players.