I think this adventure was awesome, trying to be alive inside a giant creature? That aside, there are some things that I have to question:
- I think generally, your job is to make the rooms interesting (or at least help the GM with that), so that players are rewarded for adventuring. Because if not, can't they just go up from the way they come before? I think the lines of "Don't linger on this room, it's boring", or "Nothing is keeping the adventures in this room, they can move at any time" feels reductive.
- The mechanic for RT1 is interesting, but personally this would've been better if it was gimmick for the whole belly. There's a recurring theme and players can be vigilant on which checks they should do throughout their descend.
- I do appreciate you encouraging the GM to create facts about the zetafish, but without not many information to begin with, I feel like it's just...hard for the GMs to do that. A rumour table or an information table could be nice to help GMs answer the questions about zetafish itself.
A little random encounters could help making the adventure more spicier, the only way I can see myself running this adventure would be to deliberately speedrun it. If that's your angle for minimal prep, then that's good. Anyhow, it's still a worthwhile entry, good job!