The dungeon inside a monster is a very good idea, and it seems pretty playable in one session.
I have some issues with the writing :
* outlining monsters or objects in bold or italics helps me as a GM in my session prep
* I think the room descriptions can be shorter and so we would have a little more info for the prep
* And I'd like more info on the Zeta fish, and which statblock to use for the shrimp people.
I think for lvl 1 or 2, DC 15 checks are a little hard if they are supposed to be succeeded for the most.
Great job anyway, I will use some of these ideas if my PCs are stock in a big Astral Sea monster.